You're not alone if you're worried or unsure. Most spa users (most British people in general, perhaps?) hate tipping and don't know how much to give or how to go about it. There's often an awkward moment at the end of a treatment where you're not sure whether your therapist is hovering out of concern for your comfort and well-being, or because she expects some cash. It's not practical to stuff a fiver in your robe pocket, especially if you're having a swim as part of your spa experience. That said, most therapists expect a tip for good service, and there are various ways in which you can leave one.
If you want to reward your therapists, you don't have to tip them directly; when you go to pay, you can add a tip to the overall bill. Or, you can leave some cash in an envelope at the spa reception desk when you're dressed and are on your way out. (Ah, that's what those little envelopes are for...)
A suggested rate for a tip is between 10 and 20 per cent of the treatment cost. Some spas, such as Ragdale, have a system of pool tipping: they add an optional 2-3 per cent to your total bill which is then shared amongst all the staff. A very civilised arrangement.
Of course, if you don't feel you had good service, you don't have to tip. In that case, a quiet word with the spa manager to explain why might be appreciated.