I think the main concern is the differences in CGT rules between the UK and Spain, and how it applies to your main UK residence when you sell it. I previously made a similar point on this thread:
I suspect that if you say sell your UK home before coming to Spain but then spend a sufficient amount of time in Spain during the same year to become a Spanish tax resident, then the sale of the UK home will fall under Spanish tax rules. Now I’m no expert, but I believe the rules in Spain on selling your main home are along the lines of: you don’t pay CGT if you purchase another main residence within 6 months. However if you sell your main home and then buy a cheaper home in Spain you can get taxed on the difference because you are assumed to be taking a profit.
I think Mark should be splattering this all over the front pages of the UK press. The point is people in the UK can (legally) avoid paying CGT on their UK residence if they sell it before (or after) becoming a Spanish tax resident. Fail to do so and they can get hammered. But very few people are aware of this. Before it didn’t matter, nobody checked up on it, now it’s different.