Article in today's Sunday Times, which seemed to have some elements of a "planted" story, indicated that 10 Downing Street, from their contacts with the USA, is to the fore on this rather than the DfT, and it is a now question of when rather than if.
I do hope that, if introduced, airlines (or the UK/EU governments) make it in both directions, otherwise it will be trumpeted as the US having "safer" citizens than Europe. For a country which had over 8,000 gun deaths in 2014 that is a joke.