Just to add to Genghis' comments, I'd endorse the idea of doing the NPPL (Microlight) licence first. You may find that that gives you all the cheap and fun flying which you need - the UK microlight community is very vibrant at the moment with lots going on and a very friendly atmosphere. See their bulletin board
here.
If you do want to fly something bigger then the upgrade to the NPPL SEP is very straightforward and need not be expensive, since there are very few hours required, assuming you meet the required flying standard.
Where you will hit a financial barrier is upgrading from there to a JAR PPL if you want to do night, IMC or foreign flying in a SEP aircraft. That will require at least 35 hours more training, unless you do your original NPPL training at a JAR school with a JAR instructor. By the way, while you can fly overseas on an NPPL (Microlight) licence, you can't yet on an NPPL (SEP) as far as I know.
NPPL information is
here.
For microlight information have a look at the following:
The
British Microlight Aircraft Association
Nick's Micro Pages and click on the link for 'UK 3-Axis Aircraft' to get an idea of some of the types available. Some, like the Jabiru, are available in either microlight or light aircraft versions.