In hindsight, the UK guideline was probably a good idea largely because a longer gap seems to lead to much longer-lasting immunity. Dr Campbell MD has a YouTube video in which he provides evidence to support this claim of his that the longer gap is the reason that the UK had much less of a spike in cases recently than Israel did, and extrapolates to the US.
Initially, the reasoning behind the NHS not following the manufacturers' dosing gap instructions that had been followed in the Phase 2/3 trial which demonstrated efficacy (and the US chose this conservative option) the was that x people getting 1 dose would have more societal benefit than x/2 people getting 2 doses, because it was felt that the marginal benefit of a second dose was smaller than of the first. In other words, the basis for the decision was fundamentally political, but grounded in evidence of relative impact of each shot.
The 4 UK Chief Medical Officers agree with the Joint Committee on Vaccination and Immunisation (JCVI) that at this stage of the pandemic prioritising the first doses of vaccine for as many people as possible on the priority list will protect the greatest number of at risk people overall in the shortest possible time and will have the greatest impact on reducing mortality, severe disease and hospitalisations and in protecting the NHS and equivalent health services. Operationally this will mean that second doses of both vaccines will be administered towards the end of the recommended vaccine dosing schedule of 12 weeks. This will maximise the number of people getting vaccine and therefore receiving protection in the next 12 weeks.