I think this has been covered before on this forum, but the annual registration gave you some sort of compulsory third-party insurance cover. I recall the forms having a long list of motor insurance providers on the back, from which you had to choose one. Eventually the insurance lobby had its way and it was scrapped. I'm not sure it was ever compulsory and I don't recall reg plates on speedway cars, but there are plenty of pictures of single-seaters with registration plates in Graham Vercoe's Historic Racing Cars book.
Stu