Paul Corfield wrote:

The fare is £2 per trip if you pay cash. You can purchase a "pay as
you go" Oyster card (a smartcard ticket) at Underground stations and
put cash on it and use this on the bus. If you do that then the fare
is only £1 per trip. If you make a lot of journeys over a day then
you are only charged a maximum of £3 from your cash balance on the
card. If you don't wish to hold an Oyster card but want to use the
buses a lot then the next best alternative is to buy a One Day Bus
Pass for £3.50. You can buy these from tube station ticket offices,
tube station ticket machines, ticket stops (shops that sell TfL
tickets) and machines beside bus stops in Central London.

The fare is the same regardless of how far you travel and it is
possible to go all over London by bus for not a lot of money.

Please note that in Central London where the bus stop sign is yellow
this means you must have or buy a ticket before you get on the bus.
The driver cannot sell you a ticket. On some routes there are
articulated buses (bendy buses) which allow you to board via any of
the 3 doors. Drivers do not sell and no not check tickets. If you
travel on the Heritage Routemaster services then the conductor on the
bus will check your ticket or sell you a cash fare.

If you have a PAYG Oyster card then you must place this on the reader
each time you board. The reader is either by the driver at the front
door or on a pole if getting on at doors 2 and 3 on a bendy bus.

Sorry if it all seems a bit complicated but the buses are a good way
to see London and have a good visit.

Thank you very much Paul, for these precisions.

I'll take the bus only 4 or 5 times a day, a French bus dropes us at
Central london and then, we manage by ourselves.
We will stay only one day in London.

I think I'll buy a One Day Bus Pass, as you indicated above.

Thanks again,

Michèle