One of the Lowepro Prorunner bags might do it (not sure if you could configure the
ProRunner 300AW for two bodies, but it's only just outside your budget and I have one that swallows a huge amount of medium format film gear very securely and is comfortable to wear), but just outside your budget. There are a lot of options, lying just outside your budget or between £50 and £100.
Opening up the budget to £100 would be more realistic, trying to stay under-£50 you're left with the no-name options (just do a search for "camera backpack" on Ebay and take your chances) where you'll be left a little uncertain about whether the materials, stitching, clasps, etc. can be relied upon. Or, hang around the classifieds section and pick-up a nearly-new or lightly-used bag. I know this is again providing an unasked for opinion on your budget, but we're only being realistic - the amount of gear (weight) you want to fit into a bag means you can't skimp on build quality of the bag if you don't want to be sweeping up an armful of broken glass and plastic from the pavement that used to be worth a couple of grand. Plus, you'll completely f*** your back if you try carrying this amount of gear in a bag without decent straps to distribute the weight.
Your gear, your back, your budget.. you pays your money and takes your chances..