Given the dire over-crowding on the Gospel Oak-Barking line and no immediate solution to address this (electrification, if it happens, is a good few years away and no-one is going to order more DMUs for it!), then I would suggest a deal in which TfL and the DfT under-write the refurbishment of another rake of Mk3s/DVT plus possibly the Blue/Grey commuter set of Mk3s for Chiltern. This would allow the 4 172/0s to move to LOROL, permitting every other Gospel Oak train to be 4-cars, thus buying a bit of time in terms of capacity until it is wired, whilst Chiltern operate more Mk3 sets to Birmingham, allowing a cascade of 168s and 165s to free up the 3 172 diagrams.
In addition, once Liverpool-Manchester/Wigan gets electrified towards the end of 2014, Northern will have a little more flexibility in their DMU fleet. So, 8 150s go off to FGW, with their 8 143s replaced directly at Exeter and concentrated at ATW. In turn, ATW give up their 8 153s, with the 143s replacing 150/2s on the Valleys which then replace the 153s elsewhere and which go back to Northern. Northern essentially loose 4 2-car units, but as around 10 will be displaced by EMUs, they still have a net gain.
Alternatively or in addition, Northern could swap 5 156s directly with Anglia, whose 5 153s head north. Anglia loose a micro-fleet, gain some badly needed extra coaches, although again Northern loose a little of their extra capacity gained through the spreading electrification.
Both of these shuffles remove small fleets and start concentrating the 153s with fewer operators, which makes sense in light of the 2020 DDA requirements, where I think the only way that a business case for making the 153s compliant will be to reform them as 2-car units (or even 28 3-car units with the 155s added in the mix!) with only one DDA loo per set.