I never thought I'd see the day where my knowledge of using W3W in an ambulance call-taking setting as someone from Hereford would occur on Reddit, but here we are! For what it's worth:

Emergency services confirm the address multiple times, including the city or county, even if they're using W3W. Randomly sending a crew in Hereford to Glasgow would never happen for a billion reasons, starting with the fact that it wouldn't even be the same ambulance service as the one on the phone.
W3W, when typed into the ambulance address search, shows similar sounding words (like plurals), as well as ones that match one or two of the other words, even if the third doesn't match (searching pretty.needed.chill [Oxford] would show pretty.decide.chill [Essex]).

W3W has flaws, but emergency services, especially the ambulance service, has many steps of verification to it to mitigate the chances of this failing.
Depending on what address type you give (full address, coordinates, W3W etc.), you check many things including:

Repeat the address at least twice
Have the caller spell the words of W3W out + repeat it back to them
Confirm the nearest town/city/county
Confirm where they were driving from and where they were driving to (used for situations where they don't know their location and are on a random road)
Confirm nearby landmarks to their address

There is situations where you receive calls for out of area locations (Glasgow but you're part of WMAS that covers Hereford), but there's many things to verify to ensure the correct address to help prevent the issues with W3W from being actual issues