I think he is probably a member of a Volunteer Battalion of the Royal Fusiliers (RF) in a unit that had previously been a discrete (probably Middlesex) Volunteer Rifle Corps and which after 1908 became part of the London Regiment.
That would explain the fusilier collar badges, but with a home service helmet and star, as these London based VBs who were latterly aligned with the RF were known to have worn that combination after a particular date. Changing from helmets to fur caps would have been prohibitively expensive. You can just about see what appears to be a fusilier grenade mid way along the shoulder strap too.
There was a similar thread but relating to a different VRC (the 23rd) just before 1881 here:
viewtopic.php?f=19&t=6688&p=27551&hilit=Westminster+Rifles#p27551And another concerning Tower Hamlets here:
viewtopic.php?f=19&t=5191Units that might fit the frame are:
1. 10th Middlesex Rifle Volunteers
2. 11th Middlesex Rifle Volunteers
3. 23rd Middlesex Rifle Volunteers
All the above units had their own collar badges and helmet plates before 1881, but once aligned with the RF they
gradually adopted RF associated insignia, but seem to have retained helmets (as opposed to fur caps) with redesigned helmet plates.
This was a very confusing time for the VRC and the following shows the various reorganisations of the RF aligned units that 'might' be associated with the OP's photo:
1ST.[CITY OF LONDON]BATTALION THE LONDON REGIMENT.1859 December 13th. 19th.Middlesex Rifle Volunteer Corps Formed
1880 September 3rd. Re-Numbered 10th.Middlesex Rifle Volunteer Corps
1881 July 1st. Volunteer Battalion King's Royal Rifle Corps
1883 July 1st.Volunteer Battalion Royal Fusiliers [The City of London Regiment]1908 April 1st. 1st.[City of London]Battalion The London Regiment T.F.
Royal Fusiliers[The City of London Regiment] HQ 33 Fitzroy Sq.W
2ND. [CITY OF LONDON]BATTALION THE LONDON REGIMENT.1861 January 1st. Raised as The 46th.Middlesex Rifle Volunteer Corps
1880 September 3rd. Re-Numbered 23rd.Middlesex Rifle Volunteer Corps
1881 July 1st. Volunteer Battalion Royal Fusiliers [The City of London Regiment]
1883 July 2nd.Volunteer Battalion Royal Fusiliers [The City of London Regiment]1908 July 2nd.[City of London Battalion]The London Regiment
Royal Fusiliers [The City of London Regiment] TF: HQ 9 Tufton St. SW1
3RD. [CITY OF LONDON BATTALION]THE LONDON REGIMENT.1859 December 13th.. Raised as The 20th.Middlesex Rifle Volunteer Corps 22 George St.Euston Sq.
1880 September 3rd. Re-Numbered 11th..Middlesex Rifle Volunteer Corps 5 Albany St.Euston
1881 July 1st. 3rd.Volunteer Battalion [D.C.O.]The Middlesex Regiment
1890 March 3rd.Volunteer Battalion Royal Fusiliers [The City of London Regiment]1908 April 1st. 3rd.[City of London Battalion]The London Regiment
Royal Fusiliers [The City of London Regiment] TF: HQ 21 Edward St.NW1