If I can give you my two cents not knowing your personal circumstances, what I would do, instead of paying a ton of money for an overnight with whatever you are lucky to get in Wales, is to maybe plan a little trip to London, be a tourist for the day and organise a 30/60 minute punt with some of the reputable working girls in the city. Your chances to not be disappointed are likely to be lower thanks to the many reviews on here and if things really goes tits up you can still enjoy the city and be miles away from indiscreet eyes.
Especially because you say you want it to be perfect I'd rather go for a best than settle for the rest.