Thinking I might get close to the limit this year, I looked up the exact details.
It turns out that there are a number of different limits, with one as low as 15 days!
This is because each person has to take into account their own personal circumstances, as shown in:
https://www.gov.uk/government/public...-test-srt-rdr3

So things like having accommodation available, family ties, and even visits in previous years have to be looked at.

If you have been away for over three years, it is even possible to visit for up to 120 days each year.

The criteria are very different to France and things like averages can come into play. Fortunately though the answer is simple - don't go back to UK so often

I think I've been back twice in the last 6 years or so and then only for maybe a total of 7 or 8 days..

My last trip a couple of years back and was unplanned. Essentially I had to go to escort the wife back home to Spain because she'd suffered a foot injury and would have struggled with the journey on her own.

Love has no price tag

I think the OP is referring to the UK tax residency rules. It is possible for a Spanish tax resident, living in Spain, to be deemed UK tax resident under UK tax law. However, the double taxation agreements usually sort this out between the two jurisdictions.

The double taxation rules between Spain and UK are complicated. I have to complete SA109 (Residence) form each year, which is a pain. If you are new to it, I suggest consulting a tax specialist (or a good gestor) conversant with both tax laws.

As Fred James pointed, badly worded thread title. It would be more informative if it said "UK Statutory Residence Test".
Anyone planning to leave UK should be aware of it, along with appropriate double-tax treaties.