alpha charlie you asked about the difference between BA and Virgin with respect to armed skymarshalls on board. One of the major differences is that Virgin is more heavily dependent on the North Atlantic than BA. As a result, both the company and the BALPA company committee saw the need for the pilots' concerns to be addressed before a contentious issue became a commercial one. Basically, Virgin agreed to a protocol that answers all the points that BALPA is asking for on a national basis. This may have placed the company out on a limb (it presumably had to give guarantees about government liability which may not yet have been fully clarified by the main BALPA negotiators), but it was sufficient to avoid a problem between the company and its workforce. As far as I know, neither the company nor its pilots welcome the deployment of these marshals, but both recognise that their deployment is mandated by our government and is therefore inevitable.
It's perhaps worth pointing out that the deployment of marshals is not necessarily related to any direct threat to any specific flight or destination, and that therefore the presence of marshals does not imply that a particular flight is at any unusual risk. It's also perhaps superfluous to suggest that the numbers deployed an any aircraft will be appropriate to the type and the load.