You’re better to claim a mileage allowance, it’s 40p a mile. It’s far less hassle than claimimg petrol.
Becoming a Limited company has benefits, if it goes horribly wrong the client is claiming off the company and not you which could involve you having to sell your property to cover costs. It’s a bit extreme but can happen.
Also when claiming expenses, if you don’t have a reciept don’t claim. Being VAT registered is a benefit (but a PITA) as you can claim VAT back on stuff bought for the company 😉 your computer, desk etc