I would guess that you will miss the changing seasons. Its easy to crave for non-stop sunshine, but it can get a bit much after a while IMO.
You will probably yearn for a traditional English pub, although I would be very surprised if they don't start spring up in Anglo areas of Spain (if they haven't already).
If you don't get to grips with the language of your host country, this too will probably feel quite isolating.
Also, athough we bemoan blighty, we have a strong economy here, although that's likely to change. It might be a culture shock if you move into a more spartan job market.
That said, you have to balance these possible shortcomings against what you dislike about Britain. That is quite possibly why so many thousands are leaving every year and not coming back
I'm not sure what else to say here
