I'm not a builder, or even a keen DIY-er, but I have bought a few things from Screwfix, the chain of builder's merchants
Like many retailers they ask for, and keep, your contact details; so now & again I get mailings about various offers & promotions. It's not very often, and I can live with it
But every blue moon I get a letter along the lines of 'you haven't bought anything from us for a while, so here's a £10 voucher'. Well there's nothing I really need in the way of tools or bathroom fittings, but...... they sell gas cartridges
They are butane/propane mixture, standard screw thread, and the tall ones intended for blowlamps. This would be tricky with a cartridge top burner but not a problem for me as I use a hose connected stove with a pre-heating tube
So I can get three 175g cartridges for (3 X 4,49) = 13,47 - 10 = £3-47 It all helps
They have bigger cartridges but they are longer and a bit cumbersome for camping, though I have used them when I've seen them on offer at discount stores