Sorry to hear you are having this problem.
If they sell "Miralax" in the UK, see if it will help.
When my dad was on meds that constipated him, Miralax was only through prescription, now you can buy it like other OTC meds. Anyone who I know who has used it said it was good and results were the same as one's own natural function, not like a purge, just a normal movement.
The last time I had bouts with constipation,etc. was before the idiot docs realized my thyroid was NOT normal, like I had kept saying to them, once they saw that with my bloodwork they finally started me on thryoid meds and I only get an occasional problem, the last few times were painful and even caused bleeding, yes, it can be very embarassing I'm sure. Try to get lots of water and roughage (like salads) in your tummy, it may help.
I'm wishing you lots luck and comfort with this.
It can literally be a pain in the butt.
Take care,
Roe

(((((((((((( Dear Bear )))))))))))))