I think the 91 day rule was some time since. If you did no more than 91 days in the UK per tax year then you avoided tax but then the 1/9th rule cut in and you could return for 1/9th of the time you were out of the country.
Now they have digital passports and scanners they are well placed to check!
I assume that you also have to be out of the UK for a full tax year to start the process.
As TB indicated, best to check.