I think there's a "national mood" that goes up and down over time.
There was a definite period of optimism in the UK in the 1990s, which replaced a time of general social upheaval and strife in the 1980s.
I would have put the peak era of optimism from about mid 1990s to the Millennium.
In the mid 1990s there was definitely a cool, upbeat feeling about the UK and British culture, music was one part of it there were so many good bands around and bands from different parts of the country as well which kind of gave different cities an identity. England hosted a major football tournament in Euro '96, which was also a beautiful summer, and with England playing well and getting to the semis I remember the vibe was incredible, everybody felt united following that tournament. It went beyond just football fans. There was a similar vibe during the France 98 World Cup but I felt subsequent national tournaments always had a bit of cynicism around them with non-football fans being disinterested and football fans just complaining all the time.
Tony Blair definitely rode the "Cool Britannia" bandwagon and whilst the economy generally was improving quite a lot even before he came to office, there was no doubt that the change in mood and fresher, younger feel of the country made things very difficult for John Major as the Conservatives had been in office a long time and although Major himself was quite well regarded, there was just a total vibe for change hence Blair winning a landslide of historic scale.
Princess Diana as well kind of fit "Cool Britannia", behind her vulnerable image she was very good at her own PR and was quite modern and 'cool' which made her the most interesting person by far in a dry institution like the monarchy, which is why during that period leading up to her death she was the most famous person in the world.
Also as the economy was going well, this was a good time to be young, as a young person you had reason to be optimistic about the future. Higher education was expanding (although fees had just started to come in) and generally if you got a 2:1 from a decent university you would have your pick of a few graduate jobs, could get a decent salary and then after 3 to 4 years expect a promotion, substantial salary increase and given the state of mortgage lending, find it quite easy to get a mortgage for property which in those days promised to be a very lucrative investment.
This was also the era when budget airlines and generally low cost travel was expanding which also made it an exciting time to be young, the Alex Garland book "The Beach" was written about this time and basically captured the new vibe for going to south east asia and looking for cheap holidays in paradise. The "gap year" became a rite of passage during these days.
All in all, back then there was a lot of optimism and it just generally felt like a more innocent, less serious, more fun time and people were more open to others. There was not all this hate towards immigrants or Muslims back then, or demonisation of people on the dole etc. We were less insular and just more relaxed and confident as a nation.
Two significant events changed this: first was September 11th which created a lot of new worries about security and led us down a path of wars in Afghanistan and Iraq. It took its toll on Blair who could do no wrong in his first four years but soon started to look old and troubled and became unpopular.
Second event was the financial crisis and subsequent recession and austerity. This completely changed things especially for the young. The mood of optimism went and was replaced by a lot of anxiety which the Milennials now feel.