Front seat or back seat?
In a black cab the question does not arise, because the front is for luggage. In a minicab or a pre-booked transfer, the back is the default and no offence is taken. Solo passengers sometimes sit in front to chat, which is perfectly normal, but if you are travelling alone and would rather not, sitting in the back is not rude. Nobody reads anything into it.
Do you tip a London taxi driver?
Tipping is customary but modest. Rounding up to the nearest pound, or adding ten per cent on a longer journey, is normal and appreciated. On a pre-booked fixed fare the price is already agreed, so a tip is entirely optional rather than expected. No driver will react badly if you do not add one.
Luggage is the driver's job, mostly
Most drivers will load and unload your bags without being asked. If you would rather handle a delicate case yourself, just say so. What genuinely helps is telling the operator how many bags you have when you book, because a saloon that comfortably seats four does not comfortably swallow four large suitcases.
Talking, silence and the radio
There is no obligation to make conversation. A driver who is quiet is not sulking, and a driver who chats is not prying. Either way, saying you have had a long flight and would like to rest is completely acceptable and drivers hear it every day. The same goes for asking to change the temperature or turn the radio down.
Eating, drinking and phone calls
A coffee with a lid is generally fine. Hot food is not, and most drivers will ask you to finish it outside. Phone calls are your business, though on speakerphone they become everyone's. Vaping and smoking are not permitted in any licensed vehicle in the UK.
What actually annoys drivers
Very little of what passengers worry about. It is not the chatting, the silence or the tip. It is being twenty minutes late to a pickup without a message, giving a postcode that turns out to be the wrong end of a long road, or booking a four-seater for five people and hoping nobody counts. All three are fixed by a thirty-second message before the journey.
Etiquette on an airport transfer specifically
Send your flight number when you book, so a delay does not become a problem. Message the driver once you have your bags rather than when you land, because customs can take an hour. And if your plans change mid-trip, tell the operator rather than the driver, since only the office can move a booking.