What a pathetic argument! When you read crap like this you know you're being told sop—an attempt to bamboozle the public with technicalities that don't really exist. Any commonly available high powered RF tetrode from an FM or TV station would do the job albeit with some (mostly mechanical) modifications to the transmitters. Also, it's a common practice to refurbish high power transmitting valves, there are companies that specialize in doing it.
I say that as someone who has been involved with high power broadcasting transmitters, in fact I was involved in building several FM broadcast transmitters from the ground up.
At only 198kHz—which is somewhere between one 1/500th and 1/1500th its normal operating frequency—any high power VHF transmitting tetrode would be just loafing along.
No doubt the way commerce works getting a one-off specialist job to do the conversion would be expensive (I wish I could do it, I'd quote half the price and still make a handsome profit).
If the BBC is so strapped for cash perhaps it could ask the Amateur Radio fraternity to mod the TXes for free as a public service. Then after the mods the service could recommence on half power to save costs.
Also, there are good strategic reasons why this transmitting infrastructure should be kept operational on these low longwave frequencies which I haven't got time to address here.
Suffice to say, I think this proposed closure is more a hatchet job by accountants than from any long-term strategic thinking by governments.
Damn shame governments only seem to listen to nanny goat advisors these days.