The stumbling block for me with every insurance company was "awaiting a procedure". The way round it was, as Dave suggested, getting the condition and treatment excluded from the policy. Couldn't see a way of doing this without speaking to someone on the phone, got through to Staysure first and they offered me a basic policy for under £80. It excludes anything to do with any of my declared medical conditions, including PCa.
Once I've had the HIFU and catheter removal, the treatment is complete and I will be able to tick the 'watchful waiting' (the insurance industry's phrase for active surveillance) and get a reasonably priced annual policy. I've no heart or joint issues ATM, so excluding all my pre-existing conditions may still be an option.