Overheads under FC must be calculated according to the organisations “normal” practice. Where there has never been a system of calculating overheads, an acceptable system (ask your accountant to help) must be incorporated into the financial reporting system.
Certainly travel costs attract overheads, if this is your normal practice. Clearly, any cost category (or part of a category) include in direct costs must be excluded from the overheads calculation. Subcontractors never have an addition for overheads. Overheads under FC may be per person, by area occupied, salary costs or any other method or combination.