Tesco, Sainsbury etc. all tend to be quite competitive on price, but variations can arise by the various special offers they run. I've often found, as one offer expires in on supermarket, you will find the next one takes it up. Waitrose are traditionally more expensive overall.

If you want to compare, I find "mysupermarket.com" a very handy tool. I often drive to Tesco in Lewisham, but generally I find Sainsbury's quite satisfactory, although I always visit Lidl first, to see what bargains they have in their fruit n veg, which is often much cheaper than S & T, but the quality sometimes leaves something to be desired.