The earliest recorded use of the '�' - 'L' for 'libra' - was over three hundred years ago and it was indeed written then as "20 �". Like so many other English usages, this has simply changed over the intervening years.
Morning QM y'aright? It may have changed but why only pounds and not shillings and pence? If folk changed the order, would the same folk not have changed the order of the shillings and pence?
As QM wrote, folk used to write 10�, We write 10lbs 10oz and not lbs10 oz10 and we write 10' 10" not '10 "10 so what is the difference?
We used to write 2/6 meaning two shillings and sixpence rather than two sixths so to write 2� 10 would not be confusing since it would be understood from the context.
TCL, there was a reference to a job in an issue of The Law Journal in 1885 which paid (quote) "a salary of 4� a week", so it seems the changeover was relatively recent. I wonder whether the $ sign may have influenced it. Was it always put before the figure? If so, the movement of the '�' may have been an attempt to present the two currencies in a similar format. Over to you.