"The regular battalions, that is, the battalions populated by career soldiers, operate a single regimental number sequence. A new recruit is given his number at the Regimental Depot, spends up to three months training at the Depot and is then posted to the 1st Battalion which is stationed in the UK. After 18 months to two years he is then posted to the 2nd Battalion which is serving overseas in India. His posting from the Depot to the 1st Battalion, and then from the 1st Battalion to the 2nd Battalion does not affect his regimental number which remains unchanged."
Here's an example of what I meant - and you'll find similar examples awash in WO 97, WO 363 and WO 364.