The answer calculates the the annual depreciation charge using as the total cost of the lorries less the scrap value, divided by the expected lives of 4 years (as given in the question), totalling £12,500 per year.
However the lease is a 3 year lease, and the treatment described in the notes is to depreciate the asset over the shorter of:
The lease term, or The assets expected useful life.
So, I believe it should be depreciated over 3 years at £16,667 per year.
If someone would like to check themselves and let me know if they agree that would be great.