Phil, I am not sure how many privately owned rib owners would be willing to lend you a rib for a month for this, and without a record at the end of it, the UK Rib manufacturers who are small cottage industry type builds would gain little from the publicity in return for a major expense.
You might be better off buying a Pacific Diesel rib for the trip, then selling it at the end. They are reasonably priced and are easy to modify with bigger fuel tanks and kit etc. They are also a superb sea boats for this sort of thing.
Phil Smith has one for sale on the "For Sale" page of Ribnet, give him a ring. Hope this helps and good luck with the trip for a worthy cause.