Is this a seized caliper or worn bushes?

[mobilejo]

My symptoms are:
Vibration through the steering wheel on breaking at high speed, then even after breaking the vibration continues at any speed.

Stop the car, drive off, no vibration at any speed, break at high speed at vibration starts again and stays at any speed until car is stopped for a while.

Can a seized caliper be intermittent and only seize after heavy breaking, the release when the car cools down a bit?

I am leaning to bushes as the vibration doesn't change it's frequency with speed, it's quite a low frequency vibration at both 20mph and 70mph, also no caliper/disk gets hotter than the others.

What do you guys think it is? (ruled out balancing as it does this on 2 different sets of wheels/tyres both sets balanced multiple times and one even road force)