Well in London it would be pointless as they dont collect fares anyway, outside London majority of passengers are either on free concessionary passes or weekly passes so not sure what impact it would have on revenue collected. There have been strikes by guards in the past on the railways where they have worked normally except for collecting fares.
Seems to me a far better approach as it maintains passenger good will and hits the company where it hurts without losing future business
Driving, but not collecting fares would have a much bigger effect on Mondays than any other day of the week. "Work to rule" would involve not only refusing to do Overtime but things such as not stopping to pick up or set down at/near obstructed bus stops and pulling the bus into a layby and switching the engine off as soon as Drivers Hours regulations were about to be compromised. I suppose an other way of hitting revenue would be by only driving lightly loaded routes and boycotting money spinners, but presumably "picking and choosing" would breach T&Cs and the Unions couldn't dictate which services strike breakers would cover.
It would be interesting to see what would happen with drivers from a large company going on strike, in terms of management coverage. Here in GM (and the broader North West), we have had two recent examples - one on going off course, as anounced this very afternoon.
That is First Manchester drivers at Rusholme depot striking over the massive wage differentials with other neighbouring depots. This was never going to work because the depot involved only ran three basic routes. Consequently, it was easy for management to get coverage from other depots and amongst themselves to guarantee at least 50% of journeys (virtually 100% of contracted journeys) on each of the three services would run.
Similar principle with the ongoing Northern Rail dispute with similar coverage (either directly or by "bustitution") across nearly all routes during the working day, even if nothing ran early morning or after 1900ish.
But what if say, Stagecoach Manchester went on strike and there was only enough management/strike breakers to cover 10-20% of duties spread across something approaching 100 commercial routes. How would they decide what to run?