It appears that if you aren't close to anything you can go to and try, you need to find a source where you can order one and return/exchange it if you don't like it. The only one that comes to mind over there is Thomann. Also, Andertons puts out a lot of demo videos, but I know nothing of their sales policies.
What type of "project" are you preparing for? What type of budget have you set for yourself? Are there any used basses listed in your area? I'm a fan of used equipment as instruments are built to last decades if cared for. Being new to 5 strings (not stating how much bass experience you have) used is a good way to go because the initial depreciation has been absorbed by someone else allowing you to get most, all of your, or more money out of the instrument when you go to sell it.
What I can tell you is that in the "bang for your buck" category, the brands/lines of basses that have proven to be excellent instruments and good values are: Yamaha. Ibanez. Harley Benton (Thomann's house brand). Squier Classic Vibe and one more- Sire. At least one of these brands should be readily available for you to try from somewhere.
If you feel that you need to put your hands on a instrument or try different instruments you may need to plan a day trip where you can test instruments. Not knowing the geography of England at all, I wanted to see where Andertons was and found it in Guilford, which seems to triangulate between Southhampton and Portsmouth and seems to be a little more than an hour away from Southhampton. I did a quick search and Andertons appears to have many 5 string basses in stock.