Complex sentence: When the battalion completes its tour, it returns to the mainland UK and is based at barracks in Tidworth, Wiltshire, where it is the British Army's "spearhead" battalion-an infantry battalion kept at short readiness for deployment worldwide.
Split into simpler sentences: When the battalion completes its tour, it returns to the mainland UK. The battalion is based at barracks in Tidworth, Wiltshire where it is the British Army's "spearhead" battalion-an infantry battalion kept at short readiness for deployment worldwide.