'Subway' and 'underpass' in British English

Something else Christmassy, this time spotted in Forest Hill, London:



This interested me because of the use of 'subway' to refer to what I would call an 'underpass' (namely a tunnel that enables pedestrians to cross under a road).

Googling reveals that the two terms are often used interchangeably in British English. For example, I found an article in the Barking & Dagenham Post which begins:

A 71-year-old man was beaten to the ground by a group of thugs as he walked past a notorious subway.

The pensioner was attacked near the underpass linking the Mark's Gate Estate to East Road, Chadwell Heath.

The OED Online indicates that 'underpass' originated in the US – where, of course, 'subway' is used to mean "underground railway" (again, OED Online).

I wonder whether 'underpass' is ousting 'subway' here in Britain.

So a couple of questions, mainly for our British readership: would you use 'subway', 'underpass' or both? If both, is there any difference in how you would use them?

No jokes about sandwich shops, please.

(Oh, and I appreciate that 'subway' is a shorter word than 'underpass', and therefore easier to fit on a sign.)