Not sure you can have gas canisters delivered. Certainly not by mail and most couriers I've used have them as an excluded item too.
I have no idea of that part of the world but googled outdoors Rainham kent and got camping international near Chatham, Camping and General near Canvey Island and there seems to be a few in and around Chatham / Gillingham. I have no idea where Rainham is other than kent and these are in kent sonot sure how helpful I'm being.
I would suggest checking out your yellow pages and then any places it lists. You are most likely to come across Coleman (not high performance gas in their cans) and Camping Gaz (different cans). IF you need anything better then you will probably have to detour to a store like Blacks, Millets or a specialist outdoors shop in a more hilly walking area like Peak District, SNowdonia, LAkes or SCotland in the highlands. Just have to visit a shop to buy I think,.
Rainham is near Gillingham. I forgot about camping international (I dont like going there as its mostly for caravanners) and its just down the road from me so thanks for the reminder.
Here you go, they've gone up a tad since I last bought half a case (12 cans lasts a while). They are £28.50 +P&P for 12 cans (£2.37 each + postage), from memory I think the postage was about £5 for the lot....
Link
Thanks for the link, quite a saving.