Hi,
Cardiff Bay is usually a great place, fairly sheltered & nice wide slip, but I think Wales is still closed...
Slipway Permits - CBWAC - Cardiff Rowing Centre
Maybe somewhere in Bristol harbour (just watch out for statues) ? I've only used the small slip by the pub but this one looks better:
https://www.bristolmarina.co.uk/inde...id=25&Itemid=6
There might be something at Cotswold Water Park but it's a bit of a mission finding out who owns which lakes and what's permitted, I didn't really have much success when I lived near there but you might find something:
https://www.waterpark.org/
Hope that helps a bit... I'm sure someone more knowledgeable will be along now in a minute.