Pre-WWI service aside, one reason he was one of the first soldiers sent to fight is serving with 1/8th battalion of the Lancashire Fusiliers means he was likely a Territorial Force (part time) soldier and the TF was “embodied” into the Regular Army by Royal Proclamation on 4th Aug 1914. Don’t be confused with a “service number” that didn’t exist until after WWI and regimental numbers that existed during WWI; the later changed every time a man changed regiments or from regular to reserve service. Both regular infantry & the TF used 4-digit numbers but the Lancashire Fusiliers started to reach 9999 (I think it was around 1903/4) and applied to the army for new numbers and were authorised to begin again from 0001.
That said, it’s certainly possible he had some pre-WWI military service, but whether that was regular or part-time volunteer will be difficult to establish without a service record. From an age point of view, born 1878 and assuming he enlisted age 18/21 on a “short service enlistment” of 7yrs with the colours followed by 5 yrs in the Army Reserve, means he could have been free of his agreed period of military service by 1908/1911. So when WWI began he would not have been recalled for service, but would have been free to volunteer for service.
Prior to the war the TF was greatly under subscribed, but there was surge in applicants after it began and in theory he could have volunteered either before or after the outbreak of the war; his number would certainly be indicative of his date of enlistment, but you would have to identify and note the enlistment dates of all surviving records (burnt documents and pension files) on Ancestry who served in the 1/8th battalion, but that in itself has complications.
The 1/8th battalion of the Lancashire Fusiliers was only created in Jan 1914 when it was separated from the “7th, 8th” Battalion (a double battalion of 16 companies, with its HQ in Salford) that was itself created on the formation of the Territorial Force Apr 1908; prior to that the “7th, 8th” battalion was the 3rd Volunteer Battalion Lancashire Fusiliers. So in theory he could have served as a “volunteer” before the TF was created.
As there appears not to be any surviving record for Thomas it will be difficult establishing which, if any, regiment he saw regular service with. There were men with the name T (or Thomas) Price that served in the Boer War and they appear in the Queen’s South Africa Medal Roll (available on Ancestry), they served in a variety of regiments including the Lancashire Fusiliers, but as explained above, his number will not be the same in WWI and without some other documentation it will be impossible to establish which if any is your man.
Although they will not have anything like a service record for him, the Fusiliers Museum in Bury, may be able to pin down his enlistment date from his number and any pre-war service, but I believe it may require a personal visit (or perhaps a donation) see http://www.fusiliermuseum.com/