see
www.pfa.org.uk
Basically PFA planes are either homebuilt (kits) or older designs that can't get a full CoA. Nothing wrong with either of these, and there are some rather nice ones around (and some right old dogs!).
They operate on a Permit to Fly from the CAA and are limited to day VFR - so no instrument flying, no night flying and you might have problems getting into some countries with them (I know France is ok, not sure about others, see PFA site).
So if all you want is some pleasant 'nice day' flying in the UK rather than trying to mix it with the clouds and crap weather, then they're great.
However, my advice would be to get your licence first and then start thinking about shares, you never know you might hate it!
Oh and costs are anywhere from about £20/hr to maybe £80ish for something really really nice.