W, you are right to be concerned about access to the UK PPR.
Fundamentally, periods of absence from one's only or main residence only 'count' towards the relief during periods in which there is NO OTHER PROPERTY that is also eligible for the relief (TCGA 1992 s 223 (7)).
If you have lived away from that property for anywhere between 6 and 30 years, it seems highly unlikely that you will not have gained sufficient interest in another residence - potentially anywhere in the world - during that time that the permitted periods of absence regime will apply without restriction.
Note by reference to
that HMRC's interpretation (with some justification) requires merely that there be another residence that is eligible for relief looking only at ss 222 - 6 in isolation. In other words, it is quite likely that your current (overseas) residence is eligible, and your UK (former) residence is not - unless you nominated your UK former residence up to c30 years ago, etc., etc.
Furthermore, the government intends to restrict the utility of lettings relief drastically, from 6 April 2020. On my initial reading of the draft legislation, it appears to curtail the availability of the relief, including in relation to its use/letting prior to the start date of the revised legislation. Certainly, based on the government's summary of responses to the consultation on the changes to PPR / ancillary reliefs, this appears to be the government's understanding of the draft legislation as well.
If the gain is significant, then I strongly recommend you seek the advice of a qualified professional and quickly, in case there is significant tax advantage to disposing of the property prior to 6 April 2020.