It's worth remembering that London is 10m people over a huge area. There are 1001 communities within it. So a 91% overall rate could be 91% everywhere or 99% in many places and 0% in a few. Or anywhere in between.
So the history of vacinations do become blurred. Actualy for France polio was still not eradicated as of 1988, not so in 2020 but how they count that and what the vaccine uptake was for the generations who have migrated is always going to be a factor. https://www.cdc.gov/polio/progress/index.htm
Then tourists - London does get a fair few as well, and as covid has shown, virus's dont' hang about.
polio vaccine is reasonably widespread, I don't expect that most proffeshional immigrants would be missing it. i woupd be more concerned about the recent trend of 'conciencious objection' against vaccines
GPs could do a better job of asking for your vaccine record and actually checking if its up to date though. I only found out that one of my vaccines was outdated (they need to be done again every 10 years) because I spesifically was checking my vaccines for travel.
The actual numbers seem to be here:
https://data.london.gov.uk/dataset/immunisation-rates-childr...
But I am at work so can't really look. Time for me to get a booster maybe...