I realize that Italy was an ally of Britain during the First World War so it’s also ironic that it would be Mussolini to declare war on Britain two decades later.
If I may ask for a bit of clarification on the question; are you asking for insight on the reasons why the British (government, authorities, foreign office, secret services, etc.) choose to finance Mussolini at a specific time in the war (the end of 1917) as a recent and somewhat popular research of Peter Martland provided further evidence on? Or why they felt the need to finance someone? Or why Mussolini needed and accepted such favors?
Because frankly I am really not aware of any special attention towards Mussolini at the time, that would make establishing the British position (especially) towards him quite difficult. And I am not sure there has been a substantial follow up to Martland's discovery - that was just evidence of the amount of money exchanged - expanding on the possible reasoning behind the choice.