You do need an MRI to determine if there is inflammation in your SI joints. If there is, you may have a type of inflammatory arthritis associated with Crohn's, called Spondyloarthritis. Ankylosing Spondylitis is a type of Spondyloarthritis and that does show up on x-ray. But for the earlier stages of the disease (called non-radiographic axial spondyloarthritis), there isn't enough damage to show up on x-ray, but inflammation does show up on MRIs.
Only damage shows up on x-rays and it can take 8-10 years of inflammation for damage to occur, so that's why x-rays aren't the best tests. Generally they will do an x-ray first, and if that's clear, an MRI is ordered.
Other symptoms would be joint pain, swelling and morning stiffness. Pain that gets worse with rest (sitting, sleeping) and better with movement is characteristic of inflammatory arthritis. In Spondyloarthritis, any joint can really be involved, but the sacroiliac joints are very commonly involved. Hips, knees, ankles are also often involved. Enthesitis - inflammation where tendons/ligaments insert into the bone - is also common.
SpA is associated with Crohn's and can actually cause "subclinical" gut inflammation. This is sort of like a very mild version of Crohn's - doesn't usually cause symptoms. They found that if they scoped people with SpA or AS, something like 60-70% had gut inflammation and even small ulcers, many without symptoms. Some had what they call "overt" IBD - bad enough to be classified "real" IBD.
It is very hard to distinguish between subclinical gut inflammation caused by AS and by gut inflammation caused by Crohn's, so I think generally if they found ulcers and inflammation, they would (and should) just treat it as Crohn's.
There are some treatments that are common to both diseases, like anti-TNFs.
I would also get a second opinion because I think if you have all those symptoms and visible ulcers and inflammation, it seems VERY unlikely that it's just IBS. I don't know the UK system well, but hopefully someone who is from the UK and can advise you will chime in soon.
that my inflammation could have been caused by the bowel prep and lots of people have ulcers in their small intestine...even though the inflammation they found was chronic in nature being that is been there a long time. I questioned if 'lots of people' also have all the other symptoms too? He said maybe it was IBS. I said that I thought IBS didn't cause bleeding, inflammation or ulcers, he said yes that's true usually... 
