Holiday homes 90/180 rule

[FVmouse]

IMO it is much safer to make a decision, UK or Spanish fiscal resident and do nothing to blur that line.

Not blurring the line means  not having a home in the UK or elsewhere. Not ever visiting the UK. At least for a few years.

If people are willing to do that then it's simple. The problem are the people that want to keep a foot in both

It doesn’t mean that, it means making it as obvious as possible where one is resident, something which will be clearer once immigration IT systems are fully in place (maybe not until 2022 in Spain).

If one wishes to remain U.K. tax resident then do not apply for Spanish residency in the first place, or be sure to de register as appropriate.

If one wishes to be a Spanish tax resident then simply pay your taxes to Spain. Technically, until someone becomes a permanent resident after 5 years they are supposed to spend at least 183 days in Spain each year. But if one pays one’s taxes to the Hacienda as required I am sure they will not worry about that.