Gem, sounds like your plates have been stuck on with PU sealer, a common trick played by garages for a fast fix for a small problem!!
Heres what I would do,  stuck on No plates can be removed without damage very easily, you need a legnth of MIG welding wire (1mtr) or a length of the wire used by windscreen fitters to slice out broken windscreens, attach small screwdrivers to each end of the wire, to make handles, place the wire behind the plate you want to cut off, and pull on the handles.. the wire will slice right through the sealer.. dead easy
If when you get down to your boot lid, its a mess of sealer, the best way to get this removed is to go to your friendly bodyshop, and ask them to remove the sealer with the rubber wheel that they use to remove stubborn decals, and the remnants of double sided tape, no solvents are used, and no paint damage should occur, and should take no longer than 5 mins!
Refit your new plates with 8mm thick, double sided sticky foam pads, anyone that supplies plates should be able to supply these..
hope this helps
Tom