Working in A&E dealing with Friday night stabbings and gunshot wounds, provide key trauma skills that you would just not experience within a military hospital.
Those skills are very easily obtained by detaching staff to the NHS. As a TA grunt I was attached to St Thomas's to get A&E experience for my combat medic qualification. My regular counterparts went to the Bristol RI and the big Brum A&E.
While I was at St T's student nurses from Gt Ormond St were there also, as GSOH has no A&E.