Start with Blacks -
www.blacks.co.uk
Look at Go Outdoors -
www.gooutdoors.co.uk
Take in
Decathlon -
www.decathlon.co.uk
Then there's Alpkit
www.alpkit.co.uk
You need to decide if some of the gear will ever be used for hiking or cycle camping as that will determine weather or not you will go for low prices and bulk or high prices, light weight and lack of bulk. This will impact on your sleeping bags, cookware and other sundries.
Stoves - chose between gas fuelled, liquid fuelled e.g. Camping Gaz e.g. Primus and Coleman, and meths fuelled e.g. Trangia. There's no right or wrong answer here and it's down to personal choice. Meths stoves are slower.
Sleeping bags - down are lighter, more compact and more expensive than synthetics filled ones. Alpkit down bags are highly rated though the last time I looked on the Alpkit site, there were supply problems. If never to be used cycle camping or hiking then synthetics are good value. Mummy fit are snugger and keep out drafts better than square cut bags.
Self inflating sleeping mats are compact and when inflated, very comfortable. The self inflating moniker is a tad misleading - they partially self inflate but need a lungful or two of air to complete the job. Thermarests have a good reputation though there are plenty of alternatives though some are a tad heavy.
try compiling a shortlist of gear with alternatives considered and posting it up for further advice on choice.