If you are just starting out, we would recommend buying second hand kit.  Just as many give up diving each year as take it up and the result is a thriving second hand market.  A full set could cost as little as £300, whereas a decent set of new kit could easily cost up to and beyond £1500.  If you buy second hand kit, be sure to factor in the cost of an initial service before diving with it, as using untested life support equipment without knowing if it works is obviously a very dangerous thing to do.