I didn't even know these existed until a couple of months ago when I was walking through Waterloo station and heard the automated voice talking about how a train was going to divide and which carriages you should sit in for which destinations. I forgot about this till now, but it really intrigued me. Are they common anywhere else in the UK other than on the SWML? I've never heard of them from other operators across the country before.
They used to be far more common, even just 20 years ago.
There are still a few regular ones; up here in Scotland for instance all the Glasgow to Mallaig trains split at Crianlarich with an Oban portion and vice-versa.
There is also a Sundays only Inverness to Wick which detaches a Kyle portion at Dingwall. Historically all Far North trains used to have a Wick and Thurso portion until about 20 years ago, splitting at Georgemas Junction.
Going further back, many CrossCountry services up the WCML used to have a Glasgow and Edinburgh portion, splitting at Carstairs just as the Lowlander Caledonian Sleeper still does.
In Southern Territory, many mainline trains used to divide en route, either for seperate destinations or more commonly with the front portion going ahead fast or semi-fast and the rear portion calling at all or most stations behind it.