Question Author
I'll have a look at staysure. It does seem quite mad to have to pay for several million pounds worth of medical cover when you're in the UK. My bank doesn't offer travel insurance with a regular bank account, only the ones that you pay a fee, and for us these are not worth it.
The travel insurance we usually have is the one that 'tops up' your EHIC card, or whatever they call it these days, and also has cancellation on it. They used to do a policy where you could opt to exclude your medical conditions which made it cheaper, but it would seem that they don't do it now.