It also is to do with shunting when you get to a terminus station as there's no longer any shunting locos based at the big stations. For example (and a shameless plug of my own videos!) here's Tangmere at Willesden Junction with a 47 on the rear on its way down to Euston.
[video=youtube;-zm15QjFCuc]
Then the 47 pulls the whole lot back out of Euston towards Wembley yard so Tangmere could be taken off the train and turned around in a loop
The 47 is also providing electrically supplied heat to the train. Steam locos do not have this facility and can only provide steam heat, which not all coaches are compatible with these days.
The amount of railtours that happen in the UK each week boggle the mind. The following weekend this WCRC rail tour with a 47 each end almost caught me unaware...
While waiting for this, Oliver Cromwell.
This was supposed to be an A4 but a fault with the loco meant the Britannia was swapped on at the last minute. Still, a decent replacement! As you can see with the steam escaping halfway through the train this was steam heated. Nice an atmospheric
Some DRS 47s double heading the Southampton-Edinburgh boat train
I caught all these over two weekends, and it would've been more only for it being December and the lack of sunlight in the evenings. Both were in the London area with Willesden Junction offering up loads of freight too while Oakleigh Park was the local station beside where I was staying. That was also just in London, with lots more going on around the country.
A tip for you too Anthony, as you can see in both my steam videos a Mark 2 ventilated coach is the first coach behind the loco. There was a ban on locomotives coupling straight up to Mark 1 stock for 'safety' reasons for a few years. However, I think this has been lifted recently. In my Tangmere video I believe it uncoupled with the green Mark 2 for the run around in the loop, the 47 then reversed onto the back and Tangmere and the Mark 2 coupled onto the end the 47 was originally on. The whole lot when back down to Euston to pick up passengers but it was dark by the time this all happened so I didn't get a video.
However, I believe that ban has recently been lifted.