The UK is suffering from a glut of small classes of train, that are non standard and therefore cannot be used elsewhere without staff training.
Long gone are the days of late 1970s or early 1980s where virtually all regional drivers could jump in any DMU, or a Southern region driver into any EMU as they were nearly all standard.
Whats more, most of the fleets coming free are single propulsion (pure diesel, or 25Kv only etc) and due to a disjointed National policy there are virtually no services that they can run on without either upsetting environmentalists (using diesel on electrified sections of line), or being electric when line in need isn't.
The dithering over GBR means hasn't really been any strategic plan for allocating rolling stock for about 4 years, and virtually nothing has moved around (and the few that have like 360s to Corby were generally planned at least 4 years ago). Meanwhile other plans like 707s to SouthEastern have ground to a halt due to new train problems.
There are well over 1000 vehicles on order not yet in service
(classes 196, 197, 701, 720, 805, 807, 810 etc)