That's the same Southern who are, on the suburban services, ramping down portion working in favour of keeping trains long all day and removing splits and joins, right?
This is being done to eke out more capacity from the fleet and staff, which is the wrong reason to do it. I don't oppose portion working fundamentally (I strongly support it in some cases, e.g. the Barrows/Windermeres would benefit), but simple, self-contained diagrams are vital. Then when things end up in a mess you can straighten it all out again with a couple of cancellations and some stepping-up.
With a complex web like this, you can't.
Portion working was not reduced on Southern, indeed it was introduced in the Metro area on the Tattenham Corner to London Bridge fast services, and is used to the same extent it was before at Horsham and Haywards Heath: what changed was that there were fewer attachments and detachments at the London termini and less interworking between services. Overall performance is much better, though.
Back to LNW's Birmingham services, from May we'll have:
xx15 London Euston to Crewe via Birmingham New Street (detaches Rugeley portion)
xx24 London Euston to Rugeley Trent Valley
xx49 London Euston to Liverpool Lime Street via Birmingham New Street
Birmingham International to Liverpool Lime Street
Liverpool Lime Street to London Euston (xx54 from Birmingham New Street)
Rugeley Trent Valley to London Euston (xx14 from Birmingham New Street)
Crewe to London Euston (xx33 from Birmingham New Street, attaches Liverpool portion)
Rugeley Trent Valley to Birmingham International
Of course it isn't going to work: platform workings at Euston are tightened by the fact that platforms 17 and 18 are no longer available so many reoccupation times are less than ten minutes, there is interworking everywhere, e.g. Rugeley services form Liverpool services at Birmingham International, Tring workings are formed by services from all of Milton Keynes, Crewe (Birmingham and Trent Valley), Rugeley and Liverpool: it's a mess.
I suspect the only reason this was agreed in the first place is that it reduces platform occupation times at Birmingham New Street, which is the only way to make the extra Crewe and Shrewsbury services fit at New Street, but it isn't a new concept at all - it was widely used by Central Trains until the mid 2000s (e.g. Hereford to Nottingham, Shrewsbury to Lincoln, Stansted to Liverpool) and was removed to, erm, improve performance.