I think it will eventually be rolled out to everybody, as is being done in other countries including the USA and Germany.
But I suspect it's particularly important to get an mRNA booster to the 40-49s (and older) who had astrazeneca. (Also the risk of hospitalisation and death has always been increased from 40/45, rather than 50).
uk.yahoo.com/news/covid-vaccines-wear-off-why-140948961.html
Pfizer and Moderna about 90% effective after 20 weeks.
The Pfizer and Moderna jabs, both of which are mRNA vaccines, declined only slightly in the first 10 weeks after the second dose and levelled out at about 90 per cent efficacy against severe disease after 20 weeks.
But AZ about 60% after 20 weeks.
However, a yet-to-be-peer-reviewed study found that the AstraZeneca vaccine kept falling in efficacy, with protection against severe disease dipping to 60 per cent by 20 weeks.