I'm conflicted on this one. SWMBO & myself have both had our vaccinations. That wouldn't have happened yet if HMG hadn't decided to stretch the interval between first & second doses. I can't see any scientific basis for stretching that - the clinical trials didn't look at this so there's no evidence that it won't lead to less effective immunity overall. However they obviously realised that having more people wit limited immunity results in less pressure on the NHS and, at lest in the short term, fewer deaths than would be the case if fewer people had better immunity.
The trade-off personally is that we're going to be in this intermediate state sooner at the expense of being in it for longer. The risks might be off-set if the vaccines protect against being infectious; fewer people in an infectious state reduces our risks but it's not yet known if it has this benefit.
Inevitably there'll be a percentage of people who've had a single dose taking more risks over a longer period and consequently some of them will catch COVID whilst waiting for their second dose and a few will die. That will have a perfectly foreseeable political impact but I doubt BoJo & Hancock will have looked that far ahead. It also risks having a lot of people with an even longer wait if there's any interruption to supply.
I can't imagine that this is anything more than political expediency over scientific advice.