Informing the revenue of a new business

A friend is wanting to set up a small landscape gardening business. In May of this year he bought a van and some tools but has not, as yet, carried out any actual work. My query is: should he fill in the Inland Revenue Tax form and state that his business started in May or hold off until he starts getting customers?

What advantages/disadvantages would he incur if he chose either of these options?

I'm only a CIMA Stage 2 so have no idea about Tax but would like to help him out!
Neil Wilcox