This is a sergeant of a volunteer artillery unit (part of the Volunteer Force, forerunners of what used to be called the Territorial Army).
Features which indicate without doubt that he is a volunteer are the two "efficiency stars" worn on his lower sleeve. These were awarded for every five years a volunteer was classed as "efficient". Therefore this gentleman had been a volunteer for at least ten years.
From the top, the badges on his upper sleeve are (1) "Sergeant's proficiency star", which was also unique to the volunteer force, (2) Field gun, worn by sergeants in artillery units (3) his rank badge of three chevrons or 'stripes'.
The lace on his uniform is silver and therefore shows very bright in the photo. Sergeants in the regular army artillery wore gold lace, which does tend to show a little darker in monochrome photos.
Like the regular Royal Artillery, his uniform will be dark blue with a scarlet collar. The uniform fits the 1880 to 1900 timeframe perfectly. I would place it around 1885 to 1890 based on his hair style and the lack of collar badges, but it could be a little later.
He probably belongs to the unit local to Brighton, which was the "1st Sussex Artillery Volunteer Corps", later the "1st Sussex Volunteer Artillery Corps".
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