What is ETCS and why did the 387s not have it before? Why do they need it now?
Just to add to what has been said, ETCS (or ERTMS) is the industry standard for the future.
It is already fitted to a number of high speed lines on the continent and elsewhere, and is being rolled out more or less everywhere.
It is a cab signalling system with automatic train protection (ATP) and replaces traditional "lights on sticks".
The UK was slow on the uptake, but fitted the Cambrian system (Shrewsbury-Aberystwyth/Pwllheli) some years ago as a trial.
The intention was to fit the GW main line with it, and then the ECML.
But the GW plan was canned, except that the Heathrow branch was fitted.
That means Heathrow Express needs it, and also Crossrail when it reaches Heathrow.
HEx decided to use GWR 387s instead of refitting their 332 fleet, hence the project to fit the 387s.
It means GW ATP can be retired on the branch.
The ECML is gearing up to fit ETCS from King's Cross to Stoke Tunnel (Grantham), so all trains on the southern ECML will need fitting with the equipment.
There is a test track near Hertford where trains and trackside equipment from various suppliers is tested, also NR's Old Dalby test track is fitted.
HS2 will be fitted with ETCS from the start (HS1 uses the French TVM system as is used on their early high speed lines).
Thameslink has fitted and tested it in the core section, but I don't believe it is used in normal service.