Perhaps Britain could have just built a decent 800hp diesel themselves as well as those other things, they don't need to get US stuff. If Britain and US thought the Marder was what they needed they would have built one, just like if Germany thought the Chieftain or M60 was what they needed they would have built their own.
They would have made their design, it just would have had those common parts, like rifles are all different, but use the STANAG magazine.
It's still a Chieftain, but different engine, tracks and wheels. Better armor and gun than the M60, but slower. Leo has less armor, but faster. All are made in home countries, rather than bits all put together, like the Tornado fighter was from different countries.
UK showed they couldn't made a decent diesel for the Chieftain. Maybe Leland could make a decent AVDS under licience: they sure couldn't make a decent L60. And the higher HP would be pure benefit.
Since the US wanted an APC, they make the Marder from aluminum and delete the turret, so it's light enough to swim ( a stupid requirement, being amphibious)
The point is commonality in drive train, the bits that wear the fastest.