I'd also chuck money at the landlord. I did the same think a few years ago with my private residence. I sold my flat back when the market was still good, I didn't have any references so I offered my landlord a few months rent in advance and he jumped at it.
As for being in breach of planning regulations, it does depend on a few things; how many people you have coming in and out of your flat (although if five is the magic number, you should be okay as I remember you saying you don't see that many during the week, correct me if I'm wrong), how much of the flat is used for work (which would be the whole flat in your case, so that might be a minor problem), keeping your head down so you don't attract any attention from neighbours and in turn piss them off enough to report you, and if you take on a maid/receptionist.
Remember, even if the landlord did find out, he couldn't evict you straight away anyway, even if you are in breach of your contract. He'd have to go through the proper channels, taking you to court etc., so you'd have the time to move out and find alternative premises.
It isn't actually against the law for a landlord to rent out to a lone escort who wishes to work from the premises. It is when it's rented out to more than one person. You could even take a chance in telling him what your purposes are, but that could lead all sorts of sticky situations, eg. asking for sexual favours in return, or putting up your rent, which he'd be within his rights to do.
If your budget can stretch to it, legally and for the sake of less hassle, you'd be better off working on your own. You wouldn't be breaking any laws and there's less of a chance of the neighbours finding out (less traffic, etc).