Interesting that its on the SVR, as another location with a banner signal (two, actually) with a finial is WInchcombe on the GWRS, see here:
https://depositphotos.com/294436902/stock-photo-winchcombe-gloucestershire-england-august-2019.html
They are quite recent (2019, I think) and appear to be a result of mounting the banner signals on the posts previously used by the starter signals when the latter were moved to the other side of the road bridge (that is just speculation on my part, I do not know it for certain).