Are you buying it with our without your bride to be present?
I almost purchased a real engagement ring to propose to my wife with but was talked out of it by a friend. It turned out that what I thought she would like was actually 180 degrees from what she actually liked.
I proposed using a €50 promise ring. We then had a very enjoyable experience having a ring made by a local jeweler. She loved the process and now absolutely adores the ring.
If your future wife wants a big rock then there is value to be had by going to the UAE, buying the diamond (which will be cheaper than in the UK and now you can get 5% VAT back), bring it home and have it set into a ring by a good jeweler. I know several people that have done this and saved themselves a pretty penny.