Fcuk me, I remember having to do sh1t like this at school. I paraphrased large sections of Charlie Sheens dialogue from Platoon and applied it to a 1914-1918 context, and the Teacher liked it. Creative writing served me better in that case than historical accuracy.
Anyone could be the next number either side of you. I think they're just given out in batches to each region so the next number after yours will just be given to the next person in that region who joins (the number 'next to' mine went to a lad who joined up on the same day as me at the same recruitment office. I'm Signals, he went to Royal Tank Regt, I think).
By the by, there's no dash in the middle of the numbers. They're just 2508**** (or whatever).