I’m not sure why the OP is getting such a hard time.
The BBC article states that even children who are fully vaccinated and up to date with their vaccines are being offered a booster. This includes all children between 1 and 9, so children who have only recently completed their full vaccine schedule.
www.bbc.co.uk/news/health-62492784
The question the OP is therefore asking is IMO reasonable: are children aged 1 to 9 who, under normal circumstances, have had their first vaccinations but not yet had a booster, still at risk of polio then? And the answer I guess is yes.
OP, I’d say it was about risk. Usually, we have very very low risk of contracting polio. Therefore that v v low risk combined with the presumably also v v low risk of a vaccinated, un-boosted, child getting polio if exposed, means there is no need to boost children before 10. Now that the risk of getting polio has increased in London, it’s been decided it’s a sensible precaution. But the risk hasn’t increased elsewhere, so they are only doing London.