If you are nervous, you can also take some consolation from the fact that if you use a UK Credit Card to make your purchase, you have some comeback against the card issuer if it turns out that what you bought was not in fact what you wanted.

A note about GIA certification, many of the bulk dealers play around the margins of GIA for borderline cases since the value of a stone may be very different if the colour is graded as H rather than I for instance (if memory serves). If you submit the same stone enough times for GIA certification, you can get different results, so the GIA cert itself should not be 100% relied upon but should act as a guide.

Since UK Jewellery prices (especially in the high street) can be a total rip-off it would be hard not to find significantly better value outside the UK.