When you say the 'mint office', do you mean those 'London' people that like people to think they have something to do with the Royal Mint in Wales? If so, ignore whatever they say and have no dealings with them unless you want to end up receiving a lot of rubbish coins on approval for the rest of your life! I have a blank older (large) 50p blank somewhere, but I believe it does have a rim. Blank planchets are usually pretty inexpensive because although they should never leave the mint in that state, they are (in my eyes) not really errors, but just fairly uninteresting bits of metal that have missed most of the minting processes. Value will be under £5.00 I suspect.