I suggest the relevant legislation was included in the Trading with the Enemy Proclamation, No. 2 [1914, No. 1376], made on 9 September 1914. Clause 5 states:
- From and after the date of this Proclamation the following prohibi-
tions shall have effect (save so far as licences may be issued as hereinafter
provided), and We do hereby accordingly warn all persons resident, carry-
ing on business, or being in Our Dominions —
(1) Not to pay any sum of money to or for the benefit of an
enemy.
Similar provision may well have been included in the first such Proclamation, and restated in subsequent legislation, including the Trading with the Enemy Acts.
Unfortunately, the legislation does not appear to be available through the UK Government website (probably as it has been repealed).
Charles Henry Huberich published a review of the international laws relating to trading with the enemy in 1918 (https://archive.org/details/cu31924016952636), from which the above was sourced.
Paul Saunders