Hi Garnet,
I can see how this is confusing for you, if you haven't bought jewellery before!
One advantage of a high street jeweller is that the sales staff can assist with things like this; the downside is that there is less vareity and the prices are higher. Even if you decide to buy off the TV, I would recommend looking at some jewellery websites first to get an idea of what is a available. (Gems TV also have a lot of jewellery shown on their website
www.gemstv.co.uk - much more than TCJ - so I would definitely recommend looking there.)
My best advice would be to take a look at the kind of jewellery your girlfriend already wears, so you will know what she likes - does she often wear a ring, pendant, or bracelet? Does she wear gold or silver? (Silver is a fraction of the price o gold, by the way, so you will get a lot more for your money but it may not seem as "special").
As for price, you could be spending anything from about £15 to thousands, so if you don't have a specific item in mind, a good starting place would be to decide roughly how much you want to spend, and then go from there.
If all else fails, ask her! I know you want it to be a romantic surprise, but jewellery is a very personal thing, and your girlfriend would enjoy helping to choose the perfect piece which she loves as a present from you.
Good luck!