"I see they have activist judges in Great Britain too."
I hardly see how you get that from this particular judgement. Entirely the opposite in fact.
The judge was very sympathetic to the claim, as he stated in his ruling, but was only able to apply the law as written by the legislature which mandates that the trustees may only relinquish control of part of the museum collection in specific circumstances (you can find the British Museum Act to read online if you wish).
I see 'activist judge' is now being used as a synonym for 'any judge who comes to a verdict with which I personally disagree'.