The sooner some selfish people in this world start to learn that this is a war the better.
My parents in the Second World War, made tremendous sacrifices to both fight and to help in the home front for nearly SIX YEARS in the battle against people who would take away all the rights we have to travel.
A few months sacrifices by people today would really help the battle against the virus.
Calling this a war is very disingenuous (especially comparing it to WW2). Your parents - and everybody else next to them on the lines - were indeed fighting against a human enemy that were taking away our rights not only to travel, but many other liberties, and of course - most seriously - the right to a fair society no matter who you are (and given what the current rules are doing for people with disabilities, it is horrifying to see all this forgotten). The virus is a natural thing that isn't taking away our rights; the government's restrictions are doing that, and this was the authoritarianism they were battling against. Also war was killing mostly young people; the virus is mostly killing people who - sadly but frankly - are of the age they would expect to die anyway. People who fought in the war are probably aghast at what is happening now, and probably would be a bit offended if anybody was to make the comparison.
Not to mention, in the actual war, people were mostly free to go to the pub, attend a play, see friends in their houses, etc. despite the ever present threat of a bomb falling on their heads. I guess the best comparision between the war and now is the blackouts, in terms of a significant restriction that was needed and everybody tended to abide by, with fines if you didn't. But that was reasonable.
Specifically on travel, note that long established pandemic guidance by the WHO said it did not recommend border closures, as the effects would be negligible and the harms would outweigh the benefits.
You mention Australia later on, but it is a bit easier to implement down there. Firstly, it is not really a 'hub' country like, say, many European countries (including the UK) and the US are, so closing borders is a bit easier economically and logistically as it is akin to 'the end of the line'. Secondly, although many families do split between Australia and other places, they tend to be very far away and so visits tend to be every 2-3 years anyway, so the effect on the average person who travels to see people is less as they are mostly used to long periods apart and the family structure will cope with it. In the UK, and Europe, so many families have relocated across the continent but are a quick flight or Eurostar ride away, so they still have regular meets and support. People in that position are going to be really affected by any policy that makes international travel mostly impossible. Finally, I think it is safe to say that in Australia people can cope without going away mentally for a while - the weather is mostly nice, and homes have lots of space (having lived there myself, even in the Melbournian winter, I can attest to that). Here in the UK, with such dark days, cold weather, and many people living in places without much space both indoor and outdoor, not having a trip abroad can be a huge affect on them mentally. Not to mention that good mental health and vitamin D from sunnier climes (if that's where you choose to go) is great for fighting viruses at an individual level...
As I've said I travelled a bit last year. You might see this as 'selfish', but I followed every rule in place, and even being doubly sure in terms of paying for extra tests, etc. I managed to bring a bit of income to people who were really struggling, and to be honest my travels in autumn have meant I've been in a much better place mentally than I otherwise would have been right now, although I'm rapidly losing that.
I am happy to go by quarantine rules, and even letting the government in by a Polish-style app, but being forced to pay for a hotel is a step too far. And it's basically going to kill off so many tourism-related businesses in the UK; British people on staycation will not make up for a lot of that (especially as you tend to spend much less travelling in your home country than you do abroad).
And on your final point, everybody has made 'a few months sacrifice' already - plus a lot more...