Looking at the WWW approximately 140 Plus Six cars have been registered in the UK since 2023 so it would appear to be that new cars registered between 2023 to 2025 were only replacing the written off ones, just need to find out how many were registered new in that time.
howmanyleft.co.uk gives you that info, whether it is to be trusted is another matter.
New registrations:
2024 - 1
2023 - 4
2022 - 42
2021 - 41
2020 - 50
2019 - 32
Total - 170
I'm almost certain there would have been more registrations in 2023 and 2024, what about the Pinnacles for starters.
So, diving further in to that site, it seems there are two records for the Plus Six: Plus Six Auto and Plus Six, the Plus Six Auto figures are those shown above. The Plus Six figures are:
2025 - 13
2024 - 35
2023 - 35
Total - 83
Combine the two and we have (more realistically) the true picture:
2025 - 14
2024 - 36
2023 - 39
2022 - 42
2021 - 41
2020 - 50
2019 - 32
Total - 253
Plus Six Auto licenced and SORN - 147 (Q3 2025)
Plus Six licenced and SORN - 81 (Q3 2025)
Total licenced and SORN - 228 (Q3 2025)
Therefore 253 - 228 = 25 Plus Six in the UK have been written off (or, at least, are no longer indicated as licenced or SORN).