Gastroenterology and autoimmune research scientist here....
I generally tell folks to stay off the interwebs for health advice because people are hideously ill-informed and will feed you ALL KINDS of BS, but as you're sensible and already have a GP appointment....it could be several things- what it is almost 100% NOT is an 'adjustment to UK food'....have your GP test for the following:
1. Hypothyroidism- basic blood test for TSH, T3 and T4 (they usually just do the TSH, which is enough to get you started)-if TSH is high (above 3) they'll give you replacement hormone, but also push for Hashimoto's antibody test...all hashi's is hypothyroid, but not all hypothyroid is hashi's.
2. Celiac disease(basic blood test-if positive, you win an intestinal biopsy, yay!)
If #1 is positive, you have your answer (it will be a lifelong endeavour to sort your hormone level but that's another post)...if #2 is positive, you'll be referred to a gastroenterologist. If you're in or near Oxford, that specialist will be at the John Radcliffe and do come round and say hello

If neither of those things, ask for a referral to a gastroenterologist anyway...who will then investigate lots of more complicated things and determine if you have 'run of the mill' IBS- for which diet is key (FODMAPs...ginormous pain in the ars*, but it works)- or what could be a mild form of Crohn's or colitis.
Would highly recommend not introducing anything new (i.e. supplements or probiotics) until you chat with your GP. Changing things with over the counter drugs, well, changes things...and makes it much much harder to figure out where your symptoms are coming from.
An aside regarding the posts re: fibre...a low fibre diet is strongly correlated with obesity and subsequent colorectal cancer. Eat your leafy greens and cruciferous vegetables people.
Good luck! This stuff is no fun...I both research it and live it (I've got hashi's and my husband has crohn's)...but solutions do exist!