I reckon there will be enough to do Lancashire *and* Thames Valley with some to spare (*if* the 319s do both routes).
In Lancashire there are about fifteen Northern diagrams "freed" plus fifteen TPE diagrams (most of which are the Scottish ones, which will get 350s or something "better" than a 319).
In the Thames Valley the branches will not be electrified (needing DMUs still for Greenford/ Henley/ Windsor/Eton, Marlow), IEP units will go onto the Oxford/ Bristol/ Cardiff, Crossrail will take over the Heathrow/ Maidenhead services, the Reading - Basingstoke/Gatwick lines won't be wired... not a lot of 319s required (London - Newbury, plus the Oxford slows?).
So, out of 86x 319s, maybe only half of them would be needed if they ran the Lancashire/ Thames Valley services as single units (eight coaches would be nice, of course

)