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.)
Since your light is on a different circuit than the outlet, you'll need to run a switch loop from the light to the 2-gang box using 14/3 (yes 3) cable, with black as the hot, red as the switched hot, white as a spare neutral, and the bare or green wire as well, the ground (aka EGC) :)
In the light box, you'll take the black that currently goes to the fixture hot and connect it to the black of the switch loop instead; the fixture hot then gets wired to the red wire of the switch loop. The switch loop white and green wires get wired into the existing white and green wires coming into the box.
After transposing the existing GFCI from the old 1 gang box into the new 2 gang box, you'll want to leave it alone from here on out. To wire up the switch, you connect the brass screws to the black and red wires, the green screw to the EGC, and simply wirenut off (i.e. put a wirenut on the exposed end of) the neutral on the switch loop -- it's there for future use by say a motion sensor or lit switch as per 404.2(C) (neutrals are called 'grounded conductors' in the NEC, btw, if you're a Code newbie and scratching your head at this :):
C) Switches Controlling Lighting Loads. The grounded circuit conductor for
the controlled lighting circuit shall be provided at the location where
switches control lighting loads that are supplied by a grounded general-
purpose branch circuit for other than the following:
(4) Where a switch does not serve a habitable room or bathroom
Best Answer
On a wall there is not a minimum or maximum height. The question would be how high would you like it? If close to the basin they are usually placed several inches above the counter space but again no code height restriction on the walls. The one thing is it can not be placed in a wet location like above a tub unless outside the tub zone.