I suspect in certain quarters there was a sharp intake of breath followed by some drumming of fingers on tables.
Whether or not we actually invest the required 2% of GDP and haven't cooked the books by including all sorts of other associated costs, the U.K. believes NATO to be the cornerstone of Defence going forward. I think this reliance on NATO was probably the get out of jail free card in the event of a post-BREXIT economic reset and another SDSR and the need to find quick cash to buy votes in the run up to 2020.
However, behind the bluster Trump's perfectly legitimate question of why should the US foot the bill for others when they are not prepared to help themselves and their partners by investing in NATO, if we want to rely on NATO we will have to ensure our contribution isn't smoke and mirrors. Especially if Trump does decide to refocus on the domestic front which would leave the UK as one of if not the major defence player in Europe.
Whether or not the politicians appreciate this and actually start to take Defence investment seriously remains to be seen. Maybe we need Trump to call the Government's bluff before anything changes. Until then our posture will look like an unfortunate combination of hand to mouth meets knee jerk reaction.