I think it's a case of, someone posts a tin on the bay, and it fetches £20 or so, and then there's a rush as people put theirs up to try and get the same price. That causes a glut and the price tends to fall back to its normal level for a bit. Usual price, from what I've seen seems to be about £15-£18 for a full tin of .22, and about £10-£12 for .177. Wasps and BSA Pylarms tend to do a tiny bit better, on the whole, but there is an over lap.
The South of England has 2 good things, the M1 and the A1. Both will take you to Yorkshire.