I will probably make the journey to Marks Tay on the 7th February, Ive been a couple of times before and it's usually a good spot. Picked up a TM AKS EBB last year which has been a nice, straight shooting if slightly underpowered rifle. The main challenge is it's indoor in a hall you can't test fire the guns. The same people also run a fair at the Skirmish Billericay site which allows you to take kit on the range to test if agreed with the seller.Some retailers do offer payment plans allowing you to pay in smaller interest-free chunks. Have you considered that option?
It's slightly awkward as you'd need a bus or taxi ride when you get off the train from London but there is a boot sale next month in Marks Tey Parish Hall, just outside of Colchester. Unfortunately, there's no way of knowing what will be for sale until you're there.