I have spoken to owner oferta wypożyczalni pontonów rib4rent - Sprawdź Nas.
(Sorry website is in Polish). 5,5m RIB with engine 115 cost for week only 500 GBP.
Unfortunately to offers such good prices boats are few years old (I think 5,6) but well maintain. Such size it has only 2 items (and a few 4,5m)
I even offered him that I can buy a few and rent him to rent for customers.
It be not big business and again any profit be only when cost boat+trailer be around 10000 GBP (cost of buying such RIB in Croatia 5-6 y old in Winter time).
I had similar idea for Greece and Mitikas which had free marina (I will buy RIBs and local Marine service will rent them) but Jeff (from this forum) told me "do not make business in Greece".
There are a lot of "rent boat" businesses in Croatia but average price is about 200 EUR (150 GBP) per day.
It is too expensive for Polish people so the owner of rib4rent.pl found gap in "rent - rib" business and offers them for 60-70 GBP per day.
Going back to trip UK - Greece.
You have ONLY 1400 km from Calais to Trieste !!!
And from Trieste to Patra (Peloponnese) or Igumenisa (Ionian Sea) you have ferry.
My family is 5 people so car and tow trailer even including cost of the ferry is better than airplane and rent car. Plus you can visit few places (for example one week Pylos Peloponnese, one week Ionian Sea).
But again I saw some advert here on RIB.net about rent the RIB in Lefkas.
if you find it (or you can ask for help Greek members of RIB.net) you can fly to Preveza and go to Lefkas by hired car (40 minutes or less)