There's likely to be fewer and fewer services where a single car unit is large enough, and for the handful that are, the e143/e144 is a possible solution as may be the 2-car D78 conversions.
However whilst single 153s are unlikely to have much use, They are DMU vehicles, which in the short term are in short supply. The obvious solution would be to reform them back to 155s, but that still requires each pair to have a universal toilet, which this 'rumour' suggests Porterbrook feels might not be worthwhile.
Having them all as single toiletless cars used only for strengthening services operated by a compliant unit makes sense in theory, but how long would it be before an operator faced the dilemma of sending out a pair of these in lieu of a failed unit, or cancelling the service? This also would leave the remaining 7 155s in limbo.
I propose forming 3-car 153 sets and including the West Yorkshire seven in this. Each unit could have one accessible toilet and possibly one small one. 3 car units would be perfectly adequate for routes like Derby-Crewe, Nottingham-Skegness, Hull-Scarborough, and Newcastle-Carlisle.