This is probably already explained in IvanMike's Ohms FAQ but...
Stereo power amps have ratings in watts per channel at different load impedances. You can use each channel independently of each other (i.e. 4 ohms on one channel, 8 ohms on another, etc.) without problems. Don't use a load that has an impedance (in ohms) below the minimum rating for the amp. Most solid state amps will do 4 ohms, some will do 2 ohms minimum.
Many stereo power amps can be bridged to turn them into a single channel amp with twice the power output. Please note that in all cases, the minimum load impedance goes up when you bridge... an amp that would drive a 2 ohm load in the stereo mode is now limited to 4 ohms minimum, etc.
The connectors for the output are often wired differently when using the bridge mode. Most power amps with Speakon connectors use 1+ and 1- in the stereo mode and then 1+ and 2+ in the bridged mode. READ THE AMP'S OWNER's MANUAL to make sure you're using the amp correctly.
In the bridge mode, you would connect your preamp to Channel 1 and use Channel 1's gain control.
In the stereo or dual mono mode, some amps have the capability to parallel the inputs so that you only need to run one cable from the preamp to the power amp and then you use both gain controls to balance the volume of the speakers connected to each channel.
I don't know what cabinets are available in the UK..so I can't help you there.
Hopefully this helps a little bit...