What about when one adds in this assumption: GBP/USD is stable.
(I accept it is not a guaranteed assumption, but it's pretty good, it varies only slowly, especially compared to BTC at present).
With that in mind. All one would need to do is supply some "float" capital, and push money from GBP to USD via BTC; and then take as long as you want pulling it back from USD to GBP.
As others have said, the market depth is not sufficient that one could just run round and round the loop constantly gaining 16% each time - the second time around, the same prices are not available.
I don't think there is a hugely profitable business there, but there is certainly scope for balancing the currencies correctly, which is beneficial for Bitcoin.
Anyway, I'm pleased to find that no one seems to think my maths is wrong. :-)