That’s a very broad and deep subject! The best option depends on so many things – investment / equipment, turnover, stock, VAT etc etc. Is he married (does his wife work – can he give her dividends etc etc).
My main advice would be to start with a good accountant – one that specialises in small firms / self employed.
For a ltd co then computers are deductalbe (over a few years iirc) cars are not woth it due to BIK and depending on a different set of variables he’d probably / possilbly be better off using a private car and charging mileage. IME phones are totally deductable.
Edit – most accountants are more than happy to give this initial setup advice for free: on the basis that it builds a relationship with their future client.