Hi,
I am also a brit in Doha and also like some of the home comforts, here is what I have found.
Megamart at Landmark and The Centre are good for Heinz Beans, Walkers Crisps, Twinnings Tea, they also have a fairly good range from Waitrose, also at Landmark you can buy English eggs.
Qmart at the Mall also sell English bread (yes, the real stuff)(Kingsmill and Sunblest).
Let me know if you need to find anything elso, I spend alot of the time at the shops!!!!!!
All booze only available from booze shop, good selection of everything available.