It’s almost certainly not random. The two parts of the train will have scheduled departure times and for the first departure only one half will be able to depart in the desired direction. If only one part of the train continues then the part that continues will almost certainly leave first.
But figuring that out is going to require some detective work. Trains can and do reverse during their journeys, sometimes more than once. The Liverpool to Norwich service for example reverses at Sheffield, splits at Nottingham and reverses again at Ely.
As for which cars the train will split between most trains in the UK are made up of "sets". Carriages within a set are joined semi-permanently and any splits/joins that happen during regular service will happen between sets. If you look fairly carefully it is usually fairly easy to tell the difference between joins between cars within a set and joins between sets. The ends of sets will have cabs and will in many/most cases be painted at least partly in yellow*.
* this used to be mandatory for all trains and still is mandatory for trains that don’t meet the latest standards for lights.
No, there is no general rule about how trains are decoupled.
There are a number of ways this is communicated to passengers. Announcements is the main one, but not very helpful in your case. You can ask railway staff, who will generally know, but I realize this might be difficult for you also. Passengers in the carriages generally know which part they are in, unless they are getting off the train before it divides.
I believe that there are usually also labels on each carriage saying where it is going. It’s been a while since I travelled on a British dividing train, so you might want to check that. If you have reserved a seat then the seat will of course be in the right part of the train.
Credit:stackoverflow.com‘