Hmmmm. Interesting one. A few random thoughts:
For a round Britain with no ground support - certainly diesel is much easier.
You don't get a fresh diesel boat for £20-30k - you get a nice boat that will need a few quid spent on her. Diesels like to spend your money in units of around 5k
Spending a month living on a "cabin RIB" wouldn't be my idea of fun. If I was worried about being stuck somewhere - I'd pack a tent - the experience is much the same but the tent is bigger. I have a cabin RIB btw, and I'm not knocking the experience, but the value is in the weather protection and storage (which is awesome), not the accommodation (which is sh1t)
If you want awesome, why not take the Ribeye, drysuits and a pile of jerry cans? Stay in B&B/Inns/hotels. Eat & drink like Kings! You have a month, use RIBnet to drum up some shore support - I'd happily do a fuel run for you if you showed up in Donegal (I've done it before for people I don't know), especially if there was a free lunch in it for me. I might even take you to see some local sights - you get the picture. You could get a list of helpers first and plan your stops from that.
Just one request - do it for yourself, I'd quicker help someone honestly out to enjoy themselves than for some charidee