They’re just telling people this now, as they can’t predict the future.
They are being constantly pushed for comment on the summer holiday issue, so the safe answer for Govts is to tell people not to book. Otherwise it will be a repeat of last year with people booking holidays, then something changes either before they go or while they’re away.
Last year people had holidays rescheduled and when they left the country for Portugal for example things were “ok” then they had to quarantine when they returned as the situation had deteriorated. I had sympathy for these people, as they weren’t to know, but this year the Govt. are warning people in advance. So if someone wants to go ahead and book a holiday today for August, that’s fine, but if you go to for example Tenerife, and while your gone a new variant is discovered there, don’t complain when you have to do Govt. hotel quarantine in a hotel at £1750/person. The govt are rightly telling people not to book anything, unless they can cancel/afford the knock on impacts i.e. financial and time of quarantine or cancellation if they can no longer travel.
People need to calm down on this, I expect we will be able to travel abroad this summer (maybe even this spring) but it would be irresponsible for the Govt to advise it, as then people would expect the Govt. to pick up the bill. Similar to Christmas - they told people we would have Christmas relaxation of rules, then with days to go, after people had booked expensive travel, they had to renege.