So short of buying a separate 50cc frame and 125 motor and combining them or chopping two bikes up (defeating the point of trying to save money), you can't ride or insure a 125 as a 50. A restrictor won't change anything, it may be as slow as a 50 but in the eyes of the law it's still a 125 and still illegal for you to ride.
After you’ve completed CBT, you can ride a: moped if you’re 16 or over; motorcycle up to 125cc and with a power output of up to 11kW if you’re 17 or over; You must use L plates (L or D