A direct case was not reported in the UK in recent years, but evidence of very likely polio transmission was found in sewage samples two years ago:

https://nationalpost.com/news/world/polio-virus-found-in-uk-sewage-samples-risk-to-public-low

A similar situation happened in New York where an actual case was found a month later:

https://www.reuters.com/world/us/polio-found-new-york-wastewater-state-assesses-virus-spread-2022-08-01/

The short of it is, when vaccination rates fall, Polio can be reintroduced via transmission of the live virus found in the oral vaccine, usually taken in poorer countries. If someone were to take the oral vaccine and then immediately travel to a country with lessening vaccination rates, like is currently happening in the west due to the spread of right-wing conspiracy mongering, the live virus still in the vaccinated individual has a low but not zero chance of propagating to the unvaccinated or immune-compromised population there. Samples containing these vaccine-derived viruses are found a few times per year in most places, and it’s a weaker virus so often it leads to no symptoms, but in very rare instances it does take hold with the expected effect:

https://www.who.int/emergencies/disease-outbreak-news/item/2022-DON366

Despite individual cases of polio turning up, either via direct reporting or evidence found elsewhere, it would still be correct to describe polio as being “eradicated” in these countries, at least currently. Nobody is confused by this or demands reclassification of the status of polio.