[Rockape]

The slouch hat badge looks remarkably similar in outline to the QOR Glasgow Yeomanry. In 1901 they were named:
Lanarkshire Imperial Yeomanry (Queen's Own Royal Glasgow and Lower Ward of Lanarkshire) . There is no details of badges worn by this unit (that I have seen in reference books) prior to reorganisation to QOR Glasgow Yeomanry in 1908. However, the shoulder title was QOR / GIY. The ST in the photo bears no resemblence to this. My eyes are interpreting numbers not letters.

There is another possibility. The IWM lists the unit buttons with the monogram GYC. Is it possible that the ST has those letters? If so it is a shoulder title not listed in any reference works.