Degesitering in Spain

[infoseeker]

When a person leaves Spain to go back to the UK to live, is it the number of days he has spent in Spain
That year that determines whether he has been tax resident or when he deregisters?
Eg if he deregisters from the foreign citizens register on 2 nd of july, then flies home on the third of July, is he resident for taxes that year?
Does the taxman count tax residency up to the deregistration date or the date he flies out?
If CGT exemption has been claimed due to the fact that he was tax resident in Spain, over 65 and lived in the property more than 3 years, could the taxman say he is non resident for taxes, and therefore not eligible for the exemption, or, because at the time of signing in late June he fulfilled the criteria,he is eligible for the exemption?
Finally, many do not bother deregistering when they leave Spain for good.
Is it really important to do so?