British Army rates of pay at the beginning of the First World War were set out in War Office Instruction 166 (1914). You can see the figures on the Long, Long Trail website.
Some General Regulations and Orders (GRO's) for the army at that time are informative. GRO 1887 and GRO 2240 which limited the amount of advance pay an officer could withdraw from a field cashier always specify the amount in francs (the limits were 200 francs for officers of Field Grade [colonel, lieutenant colonel, major, and captains holding an adjutant's appointment] and 125 francs for officers below field grade - provided that their agents had enough funds to match the advance).
My Gt-grandfather served as an officer in the Highland Light Infantry (HLI). His Advance pay book for 1915-16 only shows amounts in francs.
I've also seen a number of pay books showing that Privates serving with the 2nd Battalion HLI posted to the front at Guincy in 1916 were being paid in French Francs in 1916.
The Birmingham Pals Living History Association website states that the men were paid in French Francs. I've also come across a number of mentions of men being paid in Francs in memoirs and history from the first world war, for example the letters of Lionel Tennyson mentioned in The Final Over: The Cricketers of Summer 1914.
There are other cultural references from the period that record soldiers being paid in Francs. The following poem was written by Private J.J Jackson, 3rd Battalion Coldstream Guards, while convalescing in Haigh Lawn Hospital, Cheshire in December 1916.
"We are but little Coldstreams meek
We only get five francs a week The
more we do the more we may
It makes no difference to our pay
Then to the trenches we do go
And carry sandbags to and fro
Bully and biscuits is the bill of fare
For there is no Hotel Cecil there..."
Any pay advanced to the soldiers in France would be deducted from their pay later. Soldiers also often arranged for a proportion of their pay to be sent home to relatives, which would limit the amount available to be advanced to them while serving in France.
I'll see if I can find copies of the GRO's somewhere online and add a link if I do.