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.)
A standard approach to sealing is polyurethane, either brush on, rub on or spray on. If the old finish is pretty solid, a light sanding may do. If it is not adhering well or where the piece is worn and stained, a more thorough sanding is needed. In all cases, you need to wipe off all dust, preferably with a tack cloth before finishing. Wiping down with mineral spirits also helps clean before finishing.
A dust mask is always a good idea when sanding, especially if there is a chance that mold is present.
A much harder finish is marine or spar finish. While originally varnish, it is now available in a urethane formula as well.
These types are specificly formulated to hold up better in wet environments.
For almost all types of finish, two or more coats are needed for a durable protection.
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Best Answer
I believe it's 48" for a bath towel.
I just put up new towel bars in my bathroom, and my method was to hold the bar up with a towel on it and have my wife tell me if it looked right. Just to be sure, she even acted like she was drying her hands to see if it felt right too. If I remember correctly, it came out to 46". (We're both short - the next owners can either complain about it or raise it themselves.)