London is good. Flight cost is zero, and if you stay somewhere out in zone 3 or beyond accomodation and food is reasonable, and travel in from somewhere like Wembley, Stratford etc takes only minutes. Punting opportunities are almost endless.
TBH, if ALL you are going for is to punt, I doubt that you will find anywhere to beat London. I have given up going abroad purely for punting purposes. The once great hot spots of Prague, Amsterdam ,Vienna etc are no longer what they were, and with the value of the pound having sunk so low, punting abroad is no longer the bargain it once was.
I still go to Prague and Vienna fairly regularly, but not solely for the punting. Prague in particular is a wonderful place to chill out, but if all you are after is punting you would be seriously disappointed compared to London.