Might be worth shopping around if thats allowed as the fees can vary from practice to practice. My OH paid about £80 last year for his medical but some of his colleagues paid much more, some in the hundreds!!
I had mine done last week at my GP surgery as they told me that I had to have it done there, as my records were there. It cost me £110!!!!!!!!!!!!!!
I then phoned driver companies and when they asked me about medical I said I had already done it - they then told me that I could have used their own doctor (who must do a job lot of them) for £25!!!! If you look on google under HGV training, just phone up some of the driver schools and try and get it done that way.
You do need a D4 form, but to be honest, it is the most basic medical test ever - my GP spent the whole time complaining about the fact that the tests used on it were out of date, ridiculous etc...
as your fairly local to me I suggest calling Hill of Plumpton driver training, they will send you details of someone who will do the medical for £36 which is the cheapest you will find round here, but you have to go through them and the doctor is in Crawley. I've just had my medical two weeks go.