Blackpool is fine but i find that the prossie's there are very clued up to the punter tourist trap thus extra for this & extra for that, especially in the many brothels. Also the place is a shit hole (my opinion) & only fit for a day visit, its not as bad as it used to be but it still seems to be a little dirty & seedy (not in a good way) even though they do seem to be investing in the place lately.
If London is out (that would be my first choice as I've never punted there) then i would base myself in or around Manchester, everything you need is within driving distance be it seaside, culture, eating out, shopping or prossies.
(by the way if you want to shake off the OH to allow you a few hours fun time may i suggest dropping them off at the Trafford Centre, Mrs Y can get lost in there for hours
)