There are some sites out there that can get you what it's worth...You just have to do a little research. They clean, polish and repair the ring before selling it and you get a good amount for the ring. I am not sure how the ownership thing works but if they gave you the ring it should be yours. I have never heard otherwise. It would be like someone gave you a house warming gift and you move or he bought you clothing o you have to give that back etc..Do you have to give those gifts back... it was a gift but I guess every state is different. It was a gift given to you. I could go on about this but I guess you should find out what your states rules and regs are.
Sorry I digressed. Check around online though there are a few sites that can help you get what it's worth.
It sounds like the OP is in the UK (she posted the price in sterling) and there is no requirement for her to return the ring. Engagement rings are presumed to be an absolute gift (unless there was an implied or express condition that the ring should be returned if the marriage did not take place as you might find with family heirlooms).