Since Johnson has been criticizing devolution, I wouldn't be surprised to see the UK's unity weakened for a bit. That shouldn't come close to what the US are currently going through, though, especially considering the traditionally conciliatory British political culture.
However, if we're talking on a global scale, the Brexit will weaken the Western bloc, which is becoming more and more relevant now, against the Eastern bloc that's reforming with China at its head. Seeing how the US are declining and how well the EU is remaining united on the mainland, it's not crazy to say the EU will eventually become a much more significant power pole, both relatively (due to the US' decline) and absolutely.