I posted a similar question here about two weeks ago and never really got a reply. I too, am interested in upgrading to a 5 to 6m RIB with a 100hp engine. The French stuff isn't great - Zodiac Medline - yuk!, Bombard - very basic, etc.
I have an Italian Capelli 470 which I think is great, but just a touch too small. I am tempted by the Capelli 500 (actually 5.05 for what it's worth) or the 570 (a bit too big? - has to be really easy to tow and launch). We regularly cross to the islands around here, 2 adults, 2 kids and camping stuff, and it will cruise nicely at 23 knots or so, (33 max), and handles a rough sea suprisingly well for it's size.
The shops round here are starting to discount boats quite heavily now - maybe because the summer has been so poor. I could get the 570 for 24 000Euros, or the 500 for about 19 000. (With 100hp Yam).
But it would be great to get some opinions on similar sized UK boats, as these may well be better suited to our Brittany waters (similar to Cornwall).