We did this in most areas in Japan (well, it was a mixture of cutting the summer holidays by to 1-3 weeks, extended hours one or two days a week, some additional hours on Saturdays, cutting stuff like sport day practices to focus on core content... it depended on your district.)
One way or another, I think the schools have managed to get everyone caught up here. On the other hand, the closures were rather shorter here, and the schools were 100% closed during the closures-the gates were locked and nobody went inside. No teachers, no kids, absolutely nobody. The teachers did not do anything during the school closures other than post homework packages once a fortnightwith a marking sheet, because in this country parents are expected to mark and re-test the homework themselves!
Given that, it was pretty reasonable to expect them to make up the hours once school restarted, in the form of longer terms and days. The trouble is that in the UK, the curriculum was not suspended (at least not this time), and teachers have been working. THe kids themselves may not have been learning an awful lot in many cases, but the teachers have definitely been providing input this time. Working long hours too, from what I heard. So it would be hard to ask them to do a bunch extra once school restarts.
On balance, I think I prefer Japan's solution of COMPLETELY closing schools and treating the closures as a "holiday, with a holiday homework pack to do," and then keeping everyone in over the summer and for longer days etc. However, it's a bit late to do this now in the UK....