using the vin number from another frame is a ringer and if the police stop you and figure it out its "good bye to your bike" they will take it and probably crush it.
keep all receipts and when you get your new frame get the receipt for it as well, then get the sva done and you will get a Q plate reg. job done all legal and above board.
even with the weld on hard tail I had it inspected but because I had used so much of the donor bike I was allowed to keep the reg, they used to use a points system for it, don't know if they still do.