How about a service dividing/joining at Bristol? You could then have termini at London and Birmingham at one end, Wales and Devon/Cornwall at the other.
It'd be quite a bit slower than existing services though!
And yeah, joining/dividing is widely used South of the Thames but getting less common. South West Trains, for instance, don't do much of it any more on their electrified lines, having mostly phased it out outside the peak times. You still get a few trains that divide at Southampton or Bournemouth but that's just to avoid sending all 10 coaches down to Weymouth when they only need 5; the other portion terminates and turns around to form a London train. They used to run a Waterloo to Portsmouth and Poole service on Sundays, splitting at Eastleigh, but I think they were looking at splitting it into two services from Waterloo to Portsmouth and Waterloo to Poole within a few minutes of each other. Don't know if they actually did that in the end or not.
During the peaks they run oddities like a Waterloo to Poole and Fareham service splitting at Basingstoke (the Poole service calling Winchester Eastleigh then most stations to Poole, the Fareham service calling all stations Basingstoke to Fareham, also via Eastleigh), and they have quite a few services that divide at Southampton, the front portion of which is then fast to Bournemouth and on to Weymouth, the rear portion calling all stations.
On the West of England services it's a different story. Many trains longer than 3 coaches will divide at Salisbury to drop off the rear coaches, and some of them have a second portion which goes onto Bristol.
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