Since making the original post, and doing a lot of research, I finally have a plan and all my equipment.
The Ortlieb Back-Rooler Pro Plus
bags are very good. The front compartment is useful and they are light, well made and hold quite a bit (70L).
I've installed the Ortlieb QL2 Anti Theft Device
on each bag. This is a wire with a loop that can be pulled out from inside the QL2 fixings (the wire can be pushed back into the fixings so only the loop part remains poking out). They are the same thickness as my brake and gear cables and don't really fill me with much confidence. But as its purpose is only as a deterrent to a passer-by then I suppose they will be fine.
I was not sure about which size to go for, but I finally bought the PacSafe 55
. It's just tall enough to fit depth-wise into the bags with the roll top full closed. The mesh expands out to the sides quite a way. It holds all my camping equipment. I've decided to only secure that one bag. I'll put clothes, food and water in the other unsecured bag. It won't be a disaster if they're stolen. If anyone has the PacSafe 80, could you say how tall it is, the dimensions on the website only specify its size when in its carry holder. It would be good to have a slightly taller mesh.
I can't have front panniers due to carbon fibre front fork, and I don't like the feel of a handlebar bag, so I went for a Overboard Pro-Sports Waterproof Backpack
. It has a roll-top waterproof design and holds 30L. I'll put all my valuables in there and strap it over the top of my panniers. When I want to leave the bike and go into a shop I'll take the backpack and leave the panniers locked to the bike.
In total I have 100 litres. That seems more than enough and will provide storage space for any items I pick up on the tour to bring home. I'm going to test the set up at the start of April with a few days cycling around central England. I'll report back any issues I find in the field (so to say). I can provide photos if anyone's interested.