One thing I've been surprised at doing my house is that getting a plasterer is easier and cheaper than you might have thought. So, be brave.
Take down the bits of ceiling and wall that are in the way, drill proper holes in the joists and noggins for the cable runs and lay the cable in. Wire up the other end of the cable, then tape over the ends of the cable where the wall lights will go.
Next get some plasterboard -- B+Q usefully sell it in half-size sheets that fit in a car -- cut to the size of your holes (with little holes for your cables), and nail to the joists in the ceiling and the noggins in the wall. Don't worry too much if there's a few mm around the gap, it doesn't need to to be too precise.
Then have a plasterer put a "skim coat" over the top of the lot. He or she may even say it's cheaper and easier for them to do the whole ceiling or the whole wall. Stand back open-mouthed while they do the lot in a few minutes!
Oh, and I'd take @Tim Baker's suggestion of picking up the power from the switch. Given that you're in the UK, if the wiring was done in the last forty years this is going to be a pull switch on the ceiling, no?
(Usual disclaimers apply: be careful mucking about with power in the bathroom, it makes sense [and is required by the Building Regs] for a circuit that's going to a wet area to be protected by an RCD.)
Put simply, in a residential structure no light fixture you can buy in a commercial big box outlet is going to pose a problem.
Most residential circuits are 15A - some are 20A and some are 10A.
A 100W bulb on a 110V system uses just under 1A, so your light fixture would need to have a minimum of ten 100W bulbs to approach the threshhold for your house's circuits.
Plus you can get CFL bulbs which produce the same amount of light at a fraction of the wattage.
So - get what you like.
Best Answer
You'll know more when you undo the screws or thumbscrews holding the fixture onto the gangbox.
Does it go without saying here that you should only do things you understand, and that the circuit's power should be off while you're doing this?