I do think pre-Brexit, was the last time the UK was taken seriously on both the international and political scene, yes.
I don’t necessarily think Brexit is behind every single thing that has gone wrong with the UK in the last couple of years as both Covid and the war have had clear impacts of their own, BUT Brexit definitely was the beginning of the downwards public spiral of UK politicians making the choice to put their ego above the good of the country and just making worse and worse decisions as they went to save face even if there is no way to save face after so many poor decisions were purposefully made in such a short period of time.
I think pre-Brexit, the UK was a prestigious influential country with its flaws but well-regarded internationally, probably in many ways “at the top of its game.” Now, and without wanting to cause offense, it’s turned a bit into a laughing stock. It’s hard to take seriously an island-nation that has voted against its best interest and which nobody actually want or know how to lead and that seem to be each day a bit more driven to the ground.
It’s rough because Brexit has made the UK dependent on seeming appealing to other countries outside the EU and it’s very hard to feel like investing into the UK or to see the appeal in the UK or it as still competitive when things are in the state they currently are.