Unfortunately a lot of people don't realize the consequences of modifications. The bad news is there is a lot out there that aren't legal and the owners don't realize/care. The good news is you can get you vehicle legal however you will have to jump through the relevant hoops and you may end up on an age related or Q plate. I've spent years building kit cars some already registered (old/classic kit cars), some via SVA, some via the old VIC and the ibex I'm building now will need an IVA so don't loose hope just do your research.
Mike