I would see if your husband wants to go to A&E. They will only treat emergencies, they aren't generally the place to go for the diagnosis and treatment of a chronic disease like Crohn's which requires specialised gastroenterologists, but symptoms such as fainting and dehydration (he could well be dehydrated from vomitting often) and other complications that can arise from Crohn's, or whatever it is he does have, are valid reasons for going to A&E. It's possible that an emergency admission could lead to sooner tests and consultations with specialists. At the very least it might result in some respite for you both if they admit him just for rehydration and help with symptom management (painkillers if needed, meds for nausea).
There is always the possibility that A&E will just send him home for not needing immediate help, so you do have to prepare yourselves for that possibility if you do go, but it could be worth trying.
I could barely drink the contrast needed for my small intestine MRI and I was relatively well at the time and given a reduced volume to drink because I've had some of my digestive system removed. The drink is truly vile and I've wondered how on earth they expect truly sick people to manage it. He may do better with the colonoscopy prep. Once he's drunk it, the rest of the preparation takes care of itself - he'll just have to stick close to the loo, and the scope itself is easy as long as he's given enough sedation, he won't even be aware it's happening until it's over and done with.