You pay the postage to Filmdev, and if you take it to a PO (assuming any are left open these days) they'll probably charge you £3.35 or so for 1st class small packet. If you've got one of those plastic envelopes and a large letter stamp, you can take a bet that Royal Mail won't notice. Most people seem to win that bet; I've lost it 3 times so far, must have a picky sorting office.

They only charge £3 extra however, so comes to £3.95 which is not too bad, but the really annoying thing is they tend to wait until then end of the week to deliver, which is a PITA. Filmdev will pay the postage and recharge you.