Question (a) ???? Flying content apparently zero. Suggest you ask that question on the "Private Driving" forum of the Professional Driver's Rumour Network.
Questions (b) and (c), see the CAA's summary, linked to on the remuneration thread.
You can claim direct costs as travel expenses just as you might claim back a train fare. It would be naughty to claim from your employer the full direct costs but get paid a share of those by your pax, but you are never naughty, I am sure.