They do, but you have to have good income multiples and they advance the money in stage payments against valuations as signed off by the architect. So, you may get say 50% of land valuation, then more money after foundations complete, then when first floor height, then roof on, then complete - so you have to have more money up front than for a traditional mortgage.
No personal experience but I believe ecology building society will lend on self-builds, especially when there is an element of sustainability to the design.
The biggest cost by far is going to be the land (probably - unless you're in remotest scotland) and sourcing a plot's very tricky if you're anywhere near the south east at least. Not to put you off but this is something we looked into quite carefully and we just couldn't manage to afford the land and the build in the final analysis.
you won't get any money loaned for the land. at least I've never heard of it. but you can get a self build mortage. we built our house and are moved in about 2 years now. it is loaned usually in 3 stages or sometimes 4. at ground floor window level, roof on and then second fix completed. you would need to have a good bit to start with as you have to have your foundation stage and some of the block work done before you would get the first installment. also the last one won't come untill you have it finished to a liveable standard.
sorry should have added if it is a possibilty for you, go for it. it was the most frustrating, hectic and stressful thing I'vee I ever done, but so worth it to have a house that has exactly everything you would want, and you know the cost of every brick and screw that went into it. we still wake up two years after and smile at what we've achieved
You definitely need to talk to Buildstore about financing
Timber frame building is the norm in Scotland as it is speedier and less weather dependent; it can be insured as normal (it often doesn't look like timber frame from the outside); increasingly houses are being constructed in SIPS which are even quicker to build and can be very well insulated
Before getting any further you really need to do some good research on availability and prices of plots; this varies enormously around the country and the price of a titsy plot down south can set you back as much as a decent sized house with land up north. You could sign up to plotfinder or something similar to get an idea of what is around. As others have said, you usually don't get a mortgage on the plot - though there may be exceptions
Put "self build" into Google and research, research, do your sums and then add some more. As house prices go down and construction prices go up, self build isn't as financially attractive as it used to be.. but it is still great fun and the result at the end will hopefully be worth it