Hello, and welcome to ribnet!
I can't give a lot of advice with regard to the Bristol Channel, but I would think for 4 adults you would be needing a 4-ish M SIB or a 5-ishm rib (Console takes up space)
I would guess at 20 Hp minmimum for that kind of load on a SIB.
What I would suggest is get on a course first. If nothing else, they should have a varierty of boats and you can get a better feel for what might suit you. Alternative is keep an eye on the "cruises" section, people are sometimes looking for crew, again, you will get a feel for the different craft out there.
I happened to trip oer a couple of Avon SIBs on Ebay this morning, one was down your neck of the woods. Type "Avon Inflatable" into the search box to get an idea.