In Aug 1914 the Country asked for more men to fight, by Sept half a million men had enlisted.
The new army consisted of over 500 battalions and was known at the time as Kitchener's Army, the new battalions were raised as additional battalions of the regiments of the Infantry, sharing their regimental traditions.
They were numbered consecutively after the existing battalions of their regiment and were distinguished by the word "service .
eg; 1st Battalion(reg Bn) 2nd Battalion(reg Bn) the 5th TF Battalion, 7th (Service) Battalion.
In your case he joined the Middlesex Regt then moved/posted to the 1st (service) Bn Royal Fusiliers, you could not have two battalions in the same regiment called the 1st battalion, so the 1st (service) Bn Royal Fusiliers was re-name to the 8th (service) Bn RF.