That story of the last few outdated valves is nonsense.
There are lots of second hand all solid state transmitters, each one with a 400 kW carrier output. The Europe 1 site had at least 5 of them, with only 5 years in use before being discarded.
And those modern units are far more power efficient than the old valve units.
The TX operator could easily modernize the Droitwich site, but surely, the BBC doesn't want to sign a new contract for enough years to justify the upgrade.
And why no one ever mentions the 2 sync relays in Scotland ?