The one thing I would say about the British army is they do not have a reputation of being shit scared of being killed.Right now London is sitting on its hands because they cant do anything else. The UDI came about because the "loyalists" feel that London has abandoned them. The troops are confined to barracks simply because they're shit scared of being killed. The army's presence in Northern Ireland is akin to putting a plaster on a broken leg
If they are confined to barracks, I think a lack of clear orders and defined objective would be a more plausible reason.
Not wanting to escalate the conflict into a direct conflict with the Police,UDR and armed civilians.
What the army should do next will be a political decision. once the army gets their order, they will carry them out in spite of the risk of being killed.