As you know HMRC approved mileage allowance of 40p per mile for the first 10,000 business miles and 25p per mile thereafter. My suggestion is that you charge 40p per mile as a minimum plus a fixed rate for your time for travelling to/from the site. This could be at a slightly lower rate than your normal hourly rate.
In the end it is up what your client is willing to pay. I am not aware of any guidelines on this area.
I hope this helps.
I would advise you to charge the accepted rate of 40 pence per mile as most of your clients will be aware of this rate as an amount recognised by HMRC. Over and above this could be seen as profiteering and lead to ill will amongst your customers
You need to take care when invoicing your clients. If your charges state that you will charge so much per hour plus mileage then, the whole amount will be taken as income and liable to VAT if you are registered. This applies to any rate of charge you use. What you spend on motoring expenses will be a deduction against your profits. Only if you pay something directly on behalf of the client can you class this as a reimbursement and not subject it to VAT.