Email a shed load of construction companies and house builders.
Interserve will let you tender for their work.
I did it about 4-5 years ago and started to get plans and drawings sent to me nothing massive just things like primary schools, police stations & medical centres.
I didnt end up doing any because they only give you a week or so to offer your price, you had to wait up to 90 days to get paid (meaning you had to fund the job yourself), I heard they have to by law offer jobs to at least 3 different contractors and usually they had already got someone.
It was way too much hassle, most want CHAS registered and possibly LTD.
It also helps if your sister is married to the guy who knows the guy, type of thing.
I think I put 10% mark up on materials, and priced skimming so I made £0.30 per m2, it was a bit too much hassle for what it was worth, then you have snagging which you get on all new builds.
Was too big a risk for me at the time, too much outlay for not enough back.