If sending under £200 you can ask a Post Office for a certificate of posting, just as you can with any item of mail. It makes no difference whether the item is FREEPOST, postage paid by licence or physical stamps, etc. This obviously doesn't prove what's inside the package though.
It specifically recommends using Royal Mail Special Delivery Guaranteed with the appropriate insurance coverage when sending over £200 of stamps to swap. That appears to cost £10.65 for my block above. I just enquired at the Post Office whether I can redeem stamps to cover the £10.65 or have to pay cash, but didn't really get an accurate and useful answer. The guy basically said I shouldn't try because if any stamps come off the package it would cause issues.
I've also now been told by two separate members of staff that country definitives (which don't have a large Queen profile on them) didn't need to be swapped, or rather one person at the Post Office counter said they hadn't a clue and the Royal Mail helpline person said no. However,
the Royal Mail shop sells new barcoded issues of these stamps and they are mentioned as eligible in a section of the over £200 swap form (as well as olympic and international definitives) so I think the correct answer appears to be yes and I've sent a sheet of them off for swap this morning.