If your employed with SWR, you only get 5 free boxes, not 10. You only get the 10 if employed by GWR, TPE, etc!
You get travel with MTR as additional, which GWR doesn’t have. (Only existing staff in the LTV area do, but ‘special criteria’ applies.)
You also get 75% off others TOCS, day rover tickets that allow either £6 for all of GWR network or £15 for a first class travel rover on GWR. You can have up to 4 people travel with you. (As tiptoptaff has mentioned)
You also get discount season tickets (which you apply for via rail delivery group) the discount is 75%.
Priv cards (75%) is given after 3 months service. However the discount season ticket is available straight away. You can only use a priv card for yourself, not a group. Unless your family have their own priv cards.
Regarding Virgin Trains, unless a bulletin is launched ‘embargo’ travel with them (as tiptoptaff mentioned usually 2months later, when you’ve already blagged your way onto their services and find out from revenue) then you can use your box when the franchise ownership comes into force.