For someone who doesn't drive an EV, what is that in $$/mi?

EV sedans use ~ 250Wh/mile. So £0.18 is ~$0.24. So you are looking at about $0.06/mile.

Much cheaper if you get a much smaller EV (which are much more available in the UK).

For example, the Dogood Zero uses 95Wh/mile which means $0.02/mile.