Cotswold Water Park's good - I think some lakes will let you launch for a set fee - not sure which ones though as the one we use is part of a sailing club & super fussy (not worth the hassle to take the RIB there when there is sea to be played with).
Cardiff any good to you ? It's nicer than Bristol for roughly the same price if you don't count the bridge sting. It's sea but it's enclosed so practically a big & fun lake, with no speed limit
Where are you based ?