Hiya, I'm in cornwall too and I get my from the GPs (well nurse usually) but they don't do it as an NHS prescription, you'd still have to pay no matter what. I was told that "the NHS wouldn't pay for anyone elses food everyday so why should they pay for our food on liptotrim?"
I was told by my nurse the reason the price varies so much can depend on where you get it from, pharmacies charge more because they work on a "for profit basis" where as the NHS doesn't (although if you go to a doctors surgery that your not a registered patient at, they would treat you as a private patient and charge more) - apparently supply and depend also plays a factor.
I was referred onto Lipotrim as i'm due to have an operation next year and it was suggested that losing weight would be in my best interest for that which is why mines through the GP (and so they can monitor my health too).
Hope this helps a little.
Cath