According to the 1910 Baedeker guide to Belgium: "By the Convention of Paris of 1865, Belgium belongs to a monetary league with France, Switzerland, Italy and Greece. One franc, 100 centimes, 80 German pfennigs, 96 Austrian hellers, 48 Dutch cents, 20 American cents and 93/4 pence are all nearly equivalent ... The gold and silver coins of France and Switzerland, and the gold coins and 5fr pieces of Italy are also freely accepted [in Belgium]. Other silver coins frequently offered to the unwary visitor should be declined."
The guide goes on to say that English and French banknotes and English gold are accepted in all the principal towns, hotels and railway stations at their full value (1lb = 25 francs).
However, even then the UK was not a member of the Convention; my 1906 Baedeker guidebook to the UK reports: "Foreign money does not circulate in England, and it should always be exchanged upon arrival."