All enhancements vary by company and in the big operators cases, vary by depot. Some Stagecoach depots the enhancements are weekends, some are just Sundays and Bank Holidays. Some areas may have night rates for past midnight. These are also applied differently as in some cases it is applied to just the hours worked in the time of the uplift (so if you worked Friday 10pm until Saturday 2am, you would only get 2 hours at the enhanced rate), sometimes it's if you work a minute over a certain time, it becomes a 'night duty' and the whole duty is then paid at a higher rate.
As for how common is it, just look at the job adverts, it's really, really common. I know some coach firms are now even paying differently depending on the type of work. You get school rate, private hire rate and rail replacement rates.
To give a some examples, I have just searched Flixbus jobs on Indeed (link below) and the results are below (Just done for an example of how enhancements are paid, not meant to start a discussion on what is a good/bad wage)
Turners £13.50 weekdays, £15 weekends
Cymru Coaches £12.50 weekdays, £20 weekends
Bouden £14 daily
Berrys £13-15 (No more details given)
Hearns £14-17.50 (no specifics but they say depends on day and time of work)
McGills £13.50 but enhanced rate on routes over to England