Brush and English Electric became part of larger companies, they didn't go bust. EE became part of GEC (along with AEI and others), then GEC-Alstom. As part of Alstom, the site at Preston is still involved in electric traction equipment. Brush has been owned by Hawker-Siddeley, BTR, FKI and (currently) Wabtec over the last 60 years.
I agree with Bill Stanier - while it's slightly sad (speaking as someone who spends their working hours as an electronics design engineer) that the recent loco purchases are designed and built outside the UK, the reality is that if you are buying small quantities of something that you need to work 'out of the box' then you have to play safe and buy a proven product if possible. (Yes, I know the 70's have had their problems, but GE was forced to use a different diesel engine to squeeze that much power into our loading gauge - an ES44AC would need a very serious diet to fit here

)
It cuts both ways - if you want a reliable freight diesel at a sensible price then go shopping at EMD or GE in the US (because they make more of them than anyone else), if you want an electric loco then go shopping at Siemens or Bombardier in Europe (as the US passenger operators have been doing for a while). And the latest EMD US passenger diesel - the F125 - was designed by Vossloh in Spain...