I think that's poppycock - who told you that? If you have a frame and you purchase it with the log book without certain insurance classifications, there's no problem whatsoever. If you buy a used frame without a logbook or an unregistered one and then put it into a bike, providing you can identify, via a certificate of manufacture from the manufacturer (not a purchase receipt) the year of manufacture, you can get a proper registration. If you can't provide a certificate of manufacture THEN you have the problem and you may need either a single vehicle approval or q plate. If you need a certificate of manufacture and you can't get one from the manufacturer, you go to classic services to get one. Then get an MOT, insure the bike on the frame number send a cheque to the dvla as an enduro bike and bob's yer uncle..... :roll:
How do I know this? I've done it.......
Simon