"Just how much training have these "sky marshalls" had for the job? What is their background etc? How well have they been screened?
Big difference between (ex) SAS personnel and perhaps, ahem, certain private companies which are used, say, to guard some of our prisoners in the UK."
In the first instance they will be UK police officers, probably the best of the Metropolitan Police SO 19 Dept. They will have been given specialist training.
Given that in Britain the only armed law enforcement people are the police and the military when called to aid the civil power (the police), I find it hard to believe that private organisations will be called upon except as a last resort.
I suspect that if the drain on police manpower is too great then serving military personnel, probably with a special forces background (eg SAS and SBS but there are other military organisations as well) and again specifically trained for the work, may be called in to assist.
There are certain private organisation in the UK that employ former special service personnel for work in the UK and abroad (eg 'body guarding vulnerable private individuals). I suppose these people might be used in the future if the situation called for blanket 'sky marshalling', but I cannot see the UK government being too keen on this. But if their US masters demand all-out coverage on aircraft then the devil might drive.