To actually answer your question and not bow to the HN-is-reddit comment above: London draws on Kent/other surrounding areas temporarily... big cities (usually) feed into small ones. This (usually) is how it works. In the case of COVID we saw 999 response units being shipped all over the country, including call handlers (ie. England handling Scotland's calls) and response units. There are SO many plans in place for this/time-slip/major incident handling that it's mind boggling.

Source: worked for the NHS, still alive despite there being 48 hour cat 1 waits and no ambulances in the entire country.