If you are serious, buy a copy of ‘The Housebuilders Bible’ by Mark Brinkley. I think it is up to the 8th edition now. Loads of useful advice and from my experience pretty accurate (read realistic) costings. We built a house 5/6 years ago (the sixth edition was out around that time).
Accord Mortgages are well worth a look at in our experience.
And definitely not being negative – self builds are great – but your budget sounds a bit optimistic unless you can do a lot yourself.