Some of us have acquired some unpadded plastic envelopes (see somewhere earlier in this thread, I'll edit if I find the post), send the film off in them. I leave the film bare (ie not in its plastic pot) but wrap it in kitchen foil. Then post it in the plastic envelope with a Large Letter stamp on it (£0.95), posted in a post box rather than the PO. Nine times out of ten it gets there fine; occasionally they catch it. In that case it gets delayed by a few days, then they charge the lab an extra £3, so it cost you £3.95 and a few extra days versus £3.35 for first class Small Parcel. If there's more than 2, I send it Small Parcel...
EDIT... can't find the link, but it was to some fleabay seller, maybe suggested by
@excalibur2 .
BTW remember that Peak and AG (at least) have freepost, although they both charge return postage...