Ah, right. Normal bladder capacity is 400-600ml. However, people who routinely hold on to their wee can cause bladder capacity to grow to over a litre.
Didn't know this would interfere with RT though. Maybe it pushes the prostate into a position where there are too many organs at risk? I don't know what your staging and gleason is, but maybe ask if you would be a candidate for some other radiotherapy, such as brackytherapy or SBRT/Cyberknife? If your hospital doesn't do these, they may not know (although they might still give you a wrong answer), so you might need to ask to be referred to one of the UK's specialist radiotherapy centres to get the right answer (and they might be better placed to handle your special case anyway).