As I noted elsewhere, without this on a broad scale, unless there's a coordinated arrangement the main achievement of such a ban would be to either put a bullet in the heads of VS and BA [1] or significantly damage the status of LHR as a major connecting airport (and indeed potentially push some airlines to not directly operate to/from the UK) [2].

[1] If applied to UK airlines but not international carriers)
[2] If applied to flights to/from the UK regardless of carrier and not joined in on by other countries; ironically, this might save VS/BA from some of the damage from the first situation if the UK tries a "US smoking ban" approach: The odds of AA/DL/UA ditching their programs in the face of AA not generally being profitable without theirs and with WN utterly unaffected are basically zero; DL and AA could just pull out of London in favor of letting VS/BA deal with the whole mess (who knows what UA/*A would do).