If you buy you own Dexcom there is a monthly (3 sensors) or quarterly (9 sensors) subscription. I expect the NHS uses one of those.
I got a 3 month Dexcom subscription for the One+ which renews every 3 calendar months. I was looking at this today. There are 92days between my last delivery in June and the next one is September, so they must assume you use some of the 12hr grace periods to make the sensors last a couple of extra days per quarter, increasing the 9 sensors from 90 days to 92.