Zoidberg, I run the risk of overloading you with information here, but here goes.
Nearly all the EE locomotive output from the mid-50s stayed in the UK, either with British Railways or the National Coal Board (both organizations now defunct). Only a fairly small proportion are left in service now, notably the class 08, class 20 and the class 37. The ex-British Rail EE locos (all diesel-electric)were:
Class 08 (350hp 0-6-0 shunter)
Class 20 (1000hp Bo-Bo)
Class 37 (1750hp Co-Co)
Class 40 (2000hp 1Co-Co1)
Class 50 (2700hp Co-Co)
Class 55 (3300hp Co-Co)
As you may know, the UK railways were privatised back in 1993 and we are left with many private companies separately owning, operating or maintaining the fleet. It's a bit of a mess frankly.
There are two ways of looking for EE spares, either in the preservation scene or the professional market. There are hundreds of preserved locos in the UK, EE locos alone have the following organizations which own locos:
Many of these organizations (and others too numerous to mention) will possess spares, many of which were reclaimed from locos destined for scrap.
If you are particularly interested in the Class 20, here is a list of present owners of these locos:
(this site lists the owners of each individual loco in service in the UK)
(this site lists all the owners of the preserved locos)
The present main operator of the class 20 is:
They own several locos now out of service but retained for spares.
Among the professional organizations that may be of assistance are as follows:
Also in the market is the company I used to work for
they actually did the refurbishment of some of the class 20s for DRS, although I was not involved in this work. I don't think they hold any spares, they just do the refurbishment work.
Incidentally, there is also EE equipment still in service on the UK passenger EMU fleet, although it is slowly being withdrawn. There are hundreds of EE507 185kW traction motors about to become surplus to requirements.
If I can be of any further help, let me know.