I did give this matter some thought a while back.
Firstly, would you be building as an amateur or as part of a business. ?
You may find it likely that the IVA process is different between the two.
If you are going to be presenting cars for on a regular basis as an amateur built one, it will almost certainly flagged up on the systems at both VOSA & DVLA.
I think the only option would be to produce cars as a low volume manufacturer.
What sort of numbers per year would you want to build ? . Would you build to order ?
Several cars on the go at the same time would lead to a large capital outlay
One of the major issues is that of final pricing. From what I have seen of the 7 type self built car. It is often very difficult to sell a car for enough money to fully recover the initial build costs.
As far as I know, to obtain a current registration mark, the car will have to be built using all new components. Although I think you can use one fully reconditioned component provided it is to an as new spec, with documentation to prove it. ( Limited to engine, gearbox & diff I believe )
As you say though, the kit car market is reasonably buoyant, in spite of the current economic climate.
My advice, find a gap in the market & exploit it !!