the balfour declaration not the balfour law, and even if it had been a law, how would a british law apply to a US president?
on the subject of jerusalem, it seems there's a rule with eurovision that the winner hosts the contest the following year, so eurovision 2019 will be in jerusalem, which will be fun.
Next year in Jerusalem
This year, for israel (and only israel), there was an order from Eurovision not to mention the capital (Jerusalem) in the voting phase.
So, "now we go to Rome for the votes of the Italian jury"
but "now we got to Israel for the votes of the Israeli jury"