Although Crohns patients do receive infliximab and humira on the NHS they aren't actually NICE approved for this, only rheumatology, in fact the current NICE legislation recommends that infliximab is given episodically (once every so often rather than regularly) its because drugs companies have been sticking their oar in that patients now get it in that manner.
So it IS available on the NHS for Crohn's patients but there just isn't an 'offical' legislation for it - thats due to come out this year but keeps getting pushed back, it did used to be like littlemissmuffet said - they pretty much only gave it to you if it was your absolute last option, but unfortunately thats changing in a lot of hospitals now and they're starting with the 'big guns' and then working down which my doctor was saying he's trying to fight from happening in his hospital.