I work in a office on 1st floor. Gas fired boiler is fitted at one end of the office. It is not on the way to an escape route, is this aceptable to be located there?

I is difficult to say without knowing the layout of the room concerned but heating boilers are usually enclosed in a fire resisting enclosure. You should bring it to the attention of somebody in safety management.

The room is large open plan, gas boilers are fitted at one end. doors to exits are at the other end of the room. The room is always occupied during working hours and there are smoke detectors on the ceiling.

The Fire Risk Assessment is not all about means of escape it is also about preventing fire and a fire spreading. You would have to see the FRA to see what measures the risk assessor had put in place to mitigate the possibility of problems from the  gas fired boiler but if you have any concerns you should contact somebody in safety management or the enforcing authority the local fire and rescue service about it.