Middlemen? None, really - you buy through a single estate agent, who is hired by the seller. You do not have a 'buyer's agent' - and the commissions tend to be a LOT lower, 1-1.5%.
You can go directly to any bank and ask for a mortgage. I would check out pretty much any of the comparison sites. There will be an 'arrangement' fee or something similar.
You'll pay land transfer tax which is not insignificant, especially on something expensive.
I wouldn't worry about older vs newer - older tend to be built with brick. IMHO British houses don't need the same level of upkeep as North American ones can (ie, roof doesn't need redoing every 15 years!).
Within an hour of London you're, frankly, fucked. Check the train in every direction. I used to live on the Milton Keynes line and a fast train would get to Euston within half an hour I think? So check that area out - Leighton Buzzard. Tring is lovely but super expensive. LB less so.
If you live in any town you will be on natural gas. You absolutely want to heat with natural gas. I would avoid oil and electric like the plague... Most places with gas central heating will have a boiler to heat the water for both taps and radiators, storing tap water in a hot water tank. Some will have instant boilers so no hot water tank. I prefer a hot water tank but not having one saves space.
You will want to get a home inspection after putting an offer in just like in the US. You will do all sorts of surveys, just like the US. Yeah obv. avoid flood plains... nothing much to worry about round London I don't think. Mould, well, yeah the UK is kind've damp, but nothing unusual. Again, home inspector should find all that.
Bank account - no. You can go to any bank. Having an account with them doesn't matter. Your credit score does. But people aren't as 'aware' of the credit score number in the UK I don't think; you'll need employment info and all that.
