The main similarity, I believe, is in the composers. Many seem to have left KPM Music for Bruton. Alan Hawkshaw, John Charles Fiddy, Steve Gray... Which automatically makes the songs and the style very similar. What differentiates is, precisely, the difference in time. I may be talking nonsense, but KPM seems to me to be older than Bruton. Bruton started at the end of the 70s, inheriting the style of KPM tracks (as many of the composers were coming from there), but started to change a lot in the 80s.
Now, it's amazing how the two libraries complement each other. Just look at works like "SpongeBob SquarePants", and the Brazilian dubbing of "El Chavo del Ocho" and "El Chapulín Colorado".