Looks like Virgin Mobile (UK) are consolidating their legacy (no longer sold) Pay As You Go plans/tariffs.
It seems they are all being moved to the
Simply 8p plan. This is an existing Virgin Media Customer-only plan, however there are some differences to it now for the few that were already on it, e.g. texts are getting cheaper and voicemail is becoming free (it was one of the strange legacy plans that introduced paid voicemail).
Anyway Virgin Mobile PAYG customers are being texted about it. I can't say for sure it's all Legacy plans. I just know that the text has been received by people I know are on different PAYG plans/tariffs.
The move is happening on the 17th September.
More information can be found here.
https://www.virginmedia.com/help/mobile/payg-change