Welcome to OpenStreetMap and thanks for adding these speed limits.

In addition to the maxspeed=60 mph tag, as there is a national speed limit sign, you could also add maxspeed:type=GB:nsl_single 
https://wiki.openstreetmap.org/wiki/Key:maxspeed:type#United_Kingdom

It's not essential to add it and your tagging is fine as it is, but some data consumers may find the extra tag useful.