Well you can say there is certainty about the Thameslink, Crossrail and IEP cascades because they are written into contracts (or at least ITTs).
So that's a pile of 319/377/387/315/16x/225/HSTs coming free at reasonably certain dates.
From the NR CP5 plans you can also make a stab at when new electrification schemes will need new or old EMUs and allow diesel stock to be recycled.
At the other end of the scale are the lease end dates currently contracted with the ROSCOs (eg all the Voyagers are leased to VT/XC up to March 2016).
Everything else is up for grabs.
It mostly depends on the scope given to TOCs in new franchise agreements, but it is becoming apparent that they are not being given very much of that in the short-term direct awards.
Then there's the whole question of upgrades, disability regulations, ERTMS fitment etc, which may force scrappage of otherwise serviceable kit.
313s and 442s apparently come into that bracket.
Hopefully Pacers will go the same way...