Ok, well London is, I think, the only place in the country that supports a wide range of prices: bargain basement to high flyer. As for Kent, I think the rates there are similar to Sussex, so expect something like ?150 for an hour, ?250 for two. That's for outcalls, incalls cost less.
Agencies like to be able to send girls over quickly, so living outside the area can be a problem, but if you're really keen there's no harm in asking. I have a friend in Brighton who worked for
this terribly pretentious London-international agency, because she likes pretentious clients. Clients have to pay ?5k a year just to view the gallery, and then at least ?500/hour, but there are some that do it. I think my friend only ever got one bookings though (in Brussels: she flew over for an hour's booking and then they arranged a discounted overnight without telling the agency). I don't know if the agent wasn't bringing the work in in general or just for my friend, but since her commission is ?200/hour, it wouldn't matter to her if clients were sparse! She was never happy with my friend's photos and after a while my friend gave up.
I don't have an agency, from what I've heard I wouldn't want to work for any of the ones that cover my area, but it never occurred to me to find someone to find work for me when I can find it myself and keep all the money! Agencies are quite good for dipping your toe in the water, though, and there are listings online. Here's the punterlink list of
London agencies, and those
outside London. As you can see, there are many more in London. That doesn't mean you'll get more work in London though, the clients are divided up between hundreds of agencies, while the county of Kent just has a handful. I don't think you should be worrying about maximising your bookings yet though, you're only looking for a side income! Just get started, and if you're not happy with how things are going then try other options.
I just noticed that you said, "possibly my picture given on request". This may reduce the bookings you get. Everyone has a website now, there isn't really a distinction between agencies that advertise online and in print. Agencies do take girls that they promote only on the phone and certainly there are punters who'll be pleased to see the new girl (especially if they're regulars), or take whoever's working, but you won't have the opportunity to grab someone's eye while they're browsing. I don't know if agencies handle photos on request, but it is perfectly acceptable to have a photo where your face is hidden. If you didn't know that already, please look around. You only need to type "escorts" into google to see what's out there.