I live in London and would expect to pay #8-10,000 on rewire and central heating (using a good quality boiler such as Worcester Bosch). It depends on lots of things though apart from size - how easy is it to run cables and pipes in the house etc.
Just about to get cent heating fitted in 3 bed semi in SW London/ Surrey - quotes have ranged between £3,500 £5,000 ex vat. That's with worcester bosch or vaillant boilers.
Not sure about rewire cost, but think it's in region of around 2-3 grand? It will depend if you want things chasing into walls and what kinds of fittings you want, I suppose. I think it's also cheaper if you're not in residence and the property is empty.
Have to say that I would't put that sort of work on the never-never - it's an expensive way of borrowing and what happens if you find other things in the house that need immediate attention - e.g., the roof starts leaking? I think there should always be something left over for contingencies.
I was quite surprised by the lower heating quote too, but perhaps o urs isn't representative as the house in question is completely unfurnished and unlived in at the moment and the men don't have to worry about doing damage to the current decoration - carpets will be lifted, they can make holes in the walls where they need to, etc. Actually, the cheapest quote was from the person I had most confidence in (which is not how it usually works!) when I met them all. The 3500 quote doesn't in fairness include a chemical inhibitor after the power flush, we will get this on top. I'm worried now that it's too cheap....we want something that functions!