doesn't look like there's an S mint mark to me either. Half sovereigns weren't struck in Sydney in 1901 as it's not listed in Marsh. The 1901 London mint was a common year, 2,037,664 minted in London that year, also according to Marsh
The wear on your half sov looks a tad inconsistent with one on the Royal Mint (RM) web site. RM coin shows wear on the higher relief aspects of the half sov namely helmet, chest and dragon body with cape and tail still well defined....v your half sov and maybe it's the pic, but do look to have ware where the RM coin does not. https://www.royalmint.com/sovereign/all/1901-victoria-half-sovereign-ef/