This is way too low. BS 5839-1, as you know, recommends a height of 1.4m. It might not be quite as obvious, however, that it does allow a tolerance on this figure. You can reduce the height by 200mm without it being regarded a variation. So, 1.2 would be fine. This figure was chosen as representing no variation and within the tolerance as it is the height you put light switches at in dwellings to make them accessible for disabled people. I dint see the need for lower height. The code does allow an even lower height if it is likely that the first person to need to use a MCP will be wheelchair user, but this is really to cater for res care etc not a town hall. Unless you could establish that a wheelchair user is really likely to be the first person to raise an alarm, anything much lower than 1.2 would be a variation that would beed recorded and agreed by enforcing authority.