Fire brigade and health board ambulance staff do slightly different jobs. Fire brigade ambulances deal with most 999 calls and when necessary the health board ambulances back them up on larger incidents or deal with the 999 calls that the fire brigade cannot handle. Health boards deal with patient transfer as well as the 999 calls. My understanding is that new health board ambulance staff are trained to a higher standard than fire brigade ambulance staff, but fire brigade ambulance staff are fire fighters as well. Health board ambulance staff work a standard 7 day week with varying shifts.
My dad is an ambulance driver and just a word of warning, it's not a pleasant job sometimes. You need to be able to detatch yourself from what you see at work and its not a job everybody can do.