Well, a Class 1, which you need if you are going to be a professional pilot , will set you back about £400 (!)
A class 2 is cheaper but I dont know by how much, I guestimate maybe £200-300.
If you are going to progress and become a professional then you might as well get the class 1 but if you're just flying for fun then I think a class 2 is best because if you're under 30 its valid for 60 months whereas a class 1 is only valid for 12 months and you dont want to be wasting money revalidating ( about £100 I think)when it doesnt benefit you.
You can have a class 2 done by any AME ( Approved Medical Examiner) and the CAA will give you a list of these.
For a class one you need to visit an Aeromedical centre (AMC) which is at Gatwick.
Do you need to know anything else?