Have you ever recorded your worst bouts on the calendar to see if they correspond with your monthly cycle? I know you have PCOS and last I heard, weren't on BC. You may find, if you chart the pain and D and your periods, that there is a correlation.
My gyno has recommended an IUD for me. It releases hormones directly into the uterus, so it may be something that you can tolerate, unlike the regular pill. It's been in the US for 5 years, but my doctor said it has been in use in Europe for longer than that, so I would assume that it's available to you in the UK.