In an edition of The Law Journal published in 1885, a post was advertised offering "a salary of 4l. a week", but - as long ago as the 1750s - there is a recorded statement reading "To pay said sum of �54 14 0 money."
Clearly, both a lower-case 'l' or an upper-case 'L' or the stylised symbol '�' were all in use at various occasions, sometimes simultaneously it would seem, appearing either before or after the amount. At some point in the past century or so, the universal use of '�' came about presumably - like most such things - just by general usage.