Hi AllessandraB and welcome to SAAFE

There is a Where To Advertise thread stickied at the top of this board; for at least some of the London directories you really need your own website, but there are others too (Massage Republic is one that doesn't, for example).
As for rates, piece of string, how long, etc. It comes up time and time again, but only you know where you are, the sort of volume you want and who you're marketing to and how - I'd pick a middle point you're happy with when you've looked at the ads in your area and then you can always adjust up or down after you see how it goes.