210/12 Cab for MarkBass LM Tube 800

2 cabs of 8 ohms in parallel = 4 ohms
2 cabs of 8 ohm in serial = 16 ohms

If it's the size, you might want to have a look at a fEARful, too. I do not know any 2x10 capable of 500 watts handling (more experienced users may correct me).
A 12/6 or 12/6 cube will do fine I think
Since you are in the UK you might also want to have a look at BAREFACED cabs

 

For a small, single cab solution that matches pretty well to that head (powerwise and tonaly), I'd consider the Baer ML112. It is a unique 112. Nice and tight down low (which benefits the big, extended low end of the 800 series Markbass amps, and uses a very powerful woofer that can really pump out some volume. The crossover/mid driver execution is the best I've heard (warm and musical versus the rather harsh executions of others I've tried/owned). That cab will be a bit on the bright side with the inherent voicing of the LMT800 (which I owned for a short while), but the upper mid control is voiced in the right place to smooth out the top end.

For a smaller, lighter solution that would still be capable of doing a loud, moderate sized pub gig, the Bergantino CN112 is nice... a bit fatter down low in the lower midrange and smoother up top. 26 pounds or so.

Both cabs are 8ohms, allowing you to expand later on, and both can handle the wattage the LMT800 puts out nicely.

IMO and IME.

 

Most small cabs (112's, 210's) are produced in 8ohm configuration. So, the typical solid state amp will put out about 60% or so of its maximum power (assuming the typical 4ohm minimum operation). Many larger cabs (that can handle more power) are produced in 4ohm configuration (CN212, ML212, etc.), allowing the typical SS amp to put out its full power.

Getting a modular 2 x 112 (each cab 8ohms) allows you to 'scale up' both the number of drivers, and the amp power when you use one or two, depending on the gig volume requirements. Connecting two 8ohm cabs in parallel (by either daisy chaining the two cabs or running each out of one of the two outputs on the amp... same exact thing) will result in the same '4ohm impedance' as a 4ohm 212 cab.

Your head will have plenty of power to push any small 8ohm 112 or 210. The power ratings of cabs are relatively unimportant, and unless it is a really cheap cab with a very low power rating, don't sweat it regarding the output power of the amp and the 'thermal power handling' of the speaker. As long as you aren't stupid (i.e., turning up so that the cab is obviously farting or compressing and then keep playing over a long period of time), you won't have a problem

 

Per an earlier post, +1 that Barefaced produces some very nice lightweight cabs, and they are located in England. Might be a good option to contact Alex and have a cab waiting for you when you get there. One of his small 115 or 112 cabs would be very light, and be in the same performance and tone universe of the Baer ML112 or the Bergantino CN112.

 

Pretty sure OP is a Pom relocating within UK.

 

+1 Too early in the morning here Depending on what is available, the CN112, the ML112 or one of Alex's Barefaced small cabs would still be my recommendations, given the head he is using.

Edit: To put a little more 'meat' on why I am recommending those particular cabs, having owned a couple of the 800 series Markbass heads, they are quite more extended into the low bass than, for example, the LMII/III, but still have the same center point on the lower midrange control (around 400hz), which makes it hard (for me) to 'punch up' a cab with a wide, even, 'no mid bump' low end (like most of the LF loaded super 12's like the Audiokinesis and the fEARful, etc.). I found that those particular heads 'balanced out' nicely through a high quality cab that has a more punchy voice (meaning a relatively higher ratio of upper bass to lower bass, and a strong voice in the mid-midrange. The Markbass heads have a perfectly voiced upper mid control to control brightness up top.

So, this resulted in me suggesting the cabs I did. Given he is in the UK, some of Alex's 'two way/non mid driver' cabs should work quite well for him. From the clips and reviews I've heard (have not heard one 'in the flesh'), they have a nice midrange punch and controlled low end, and seem to be in the tonal universe of the Baer ML112 and Bergantino CN112.

IMO and IME!

 

Another option for the OP is the TecAmp 210 cab, which gets very good reviews. Another tight, punchy, high quality, lightweight execution that would work well with the LMT800.

 

You mostly have it (except for the last part).

Yes, your 8ohm wattage is about right (the percent of total output between 8ohms and 4ohms varies a bit between amp models, but it is usually between 50% and 60% of the full 4ohm output). When you add a second 8ohm cab, the total nominal impedance drops to 4ohms, and in your case, the 800 watts the head outputs at 4ohms is split evenly between the two cabs... 400 into each, for a TOTAL output of 800 watts.

Finally, your last question is my 'daisy chain' versus 'two speaker output of the amp' example from my previous post. Those two methods provide the exact same parallel connection.... there is NO difference between running two speaker cables from the amp versus running one cable from the amp into the top cab, and then another cable from the top cab to the bottom cab in your stack. Actually with the LM series heads with that kind of strange 'one speakon/one 1/4" speaker output, the daisy chaining is a better way to go, since you can use speakons in your entire power chain.

 

It's 20 years since I lived in London. Bars weren't very big but I doubt they got bigger since. A decent 2x10 should get you by.

 

Also consider a 115 cab. The cab could be about the same size and weight as a 210, and as it will have more surface area to the drivers, it should produce more air movement (assuming the xmax, driver sensitivity and frequency response are otherwise equal).

Here's a thread that discusses some of this stuff:
http://www.talkbass.com/forum/f15/math-speaker-air-movement-111687/

 

+1 That Barefaced 115 (I assume using either a Faital Pro or 3015HO driver) is a really nice, small, lightweight box that will give the OP a bit more volume than the 112's I've suggested without much more size and weight. Always trade-offs though, since it is harder to get a tweeter to mesh with the top end of a 15, and once you get into mid driver loaded 15 cabs, things get pretty large for the most part. For a one way (no tweeter) solution, a small 115 with one of those drivers is really nice... warm, punchy, and a nice 'paper cone brightness' up top. Nice match for the OP's head also.

 

There is loud enough to need earplugs and then there is LOUD. Plenty of GK 212 neo get blown up trying to be to LOUD. Your main concern is not the drummer but gui****s that make the drummer smash his kit.

Genre and venue size might indicate you are ok with 112, or not, depending on volume excesses from gui****s.