Your link doesn't work . . . are you asking about the tube?

When using the tube you 'touch in'/'touch out'- which means you touch your oyster to the pad on the entry gate and it opens. Then when you get to your destination station, you touch the pad to open the gate to exit.

On buses you touch a similar pad on board.

this shows how it works.

oops - THIS is how it works http://www.tfl.gov.uk/tickets/14839.aspx

The note OP refers to is about buses. Out in the suburbs you can pay the driver (at something like twice the rate it costs against PAYG on Oyster), but in the city centre they won't (to save time handling cash and arguing with people who haven't got change), so those stops are plastered with "Pay in advance" signs, and some have machines you can buy a single ticket from (at the higher, cash, rate).

Or in other words, never pay cash-per-ride.

yes I am asking about the bus- gotcha Patrick - thanks - with our 7 day Oyster we will just swipe as we get on - thanks!!

On the same lines - I have read about the Heritage Bus route - # 15 and #9 - are these still running? I would just get on at Kensington High Street to Trafalgar Square or Trafalgar to Tower Hill??

During my planning I did notice some bus routes going down Whitehall but didnt pay attention to which route.

>> I would just get on at Kensington High Street to Trafalgar Square or Trafalgar to Tower Hill??<<

The 15 and 9 are regular scheduled routes. You just get on and off in the ordinary way; but a shorter version of the route is operated using the old Routemasters that used to be the standard London bus (some people felt so nostalgic when they were replaced that TfL agreed to keep some running). I'm not sure whether it's possible to know when one of those will be along as opposed to the current standard sort of bus running the full route.