... via train with two escorts, from what looks like St Pancras (wow, that looked grim!) and going in a First Class compartment.
BR's General Appendix to Rules & Regulations, dated 1972, included the following . . .
General Appendix 1972 said:
PRISONERS AND INSANE PERSONS
Prisoners and insane persons, with their escorts, must be placed separate from other passengers.
I suspect that when Second Class accommodation on daytime main line trains became almost exclusively open saloons, but First Class could still be in compartments, it was felt preferable to put prisoners under escort into First Class compartments.
I have a vague memory of some kind of kerfuffle back in the day - maybe an irate Letter to the Editor from 'Disgusted of Tunbridge Wells' - complaining that society's ne'er-do-wells were now being provided with luxurious First Class Inter-City rail travel at the taxpayers' expense.
Presumably this type of 'non-discretionary' travel was always under some kind of specialised Travel Warrant arrangement - rather than Mr. MacKay buying everyone's tickets at the booking office window - and maybe a reserved compartment & upgrade to 1st was included
gratis by British Rail - looking after “regular customers”?
Another thing I recall reading at one point was that the single Second/Standard Class compartment in the 4-CIG Driving Trailer Composites was there partly to allow discrete/secure transportation of London's convicts to Portsmouth, at the start of their Staycation at Her Majesty's expense in the Isle of Wight's Parkhurst Hotel. I'm unconvinced how much of a factor this really was.
I've also heard that the single Standard Class compartment was provided in CIGs (and CEPs too, I think) to facilitate transport of a body on a stretcher - which accessed the compartment via a hinged, fully-opening window frame. And also that compartments were desirable on the Waterloo - Portsmouth route so MoD / Navy bods could discuss confidential defence matters during the journey. Though presumably anyone discussing secrets juicy enough to be worth overhearing would be officers travelling First Class anyway. The real explanation for provision of that one Standard compartment was probably 'none of the above' and nothing to do with prisoners.