Although there 8 Tonkiss soldiers who served in WWI, Jacob doesn’t appear to one of them. As such his certificate of service should be available in document series WO97 at the National Archives, Kew.
The series is being digitised as part of co-project between the NA & findmypast.com, unfortunately the records covering the period 1900 –1913 [the most likely for Jacob] and surname group containing Tonkiss isn’t scheduled to be online until April 2010. It is possible that Jacob left the army
Movements of Worcestershire Regiment 1899-1900
Although the situation was to change dramatically by the close of 1899, The Times of 6 Jan 1899 published a programme for intended reliefs during 1899-1900 that included - “1st Battalion Worcestershire Regt., Devonport to Guernsey and the 2nd Battalion, Bermuda to Halifax, N.S.”
From the limited information you’ve given, re Devonport, that would place Jacob in the 1st Battalion of the regiment.
Times Mon 15 May – describes a field exercise around Bovisand Fort, Plymouth that the Worcster Regt., took part.
It was the 2nd battalion, recalled from Bermuda, that were to be the first to depart for South Africa. As part of the 6th Division, they sailed from Southampton on the Tintagel Castle on 17th December 1899, and arrived at the Cape on 8th January 1900.
The 1st Battalion was part of the 8th Division and left Tilbury on the Braemar Castle on 18th March 1900 [Times Mar 19], and arrived at the Cape on 8th April [Times 10th Apr]
After tracing and typing all that I find there is only one Jacob Tonkiss in the 1901 census and he’s age 27, born Wolverhampton, Staffs, an Agent Prudential Insurance, living in Liskeard, Cornwall. There is also a 26yr old Ann Maria Tonkiss, born Liskeard, living in Liskeard. I don’t have access to view the census image but if that is your Jacob and his Wife then it could be he left the army before 1900 [ie awaiting discharge whilst in barracks in 1899], in which case you will have to look in records of discharges 1883-1900, they should be online by the end of next year.
What a pitty you didn’t give full information in your post.