There are a whole lot of regs to be considered when installing boilers in the roof space, remember it has to be maintained from time to time so needs good access. roof space boarded out, power points, lighting, a fixed loft ladder with hand rails around the opening and an appliance isolation valve outside the roof space to name a few otherwise it will not fully comply. Rearly ever found I might add, no wonder my knees are suffering doing repairs whilst balancing on a 2" wide joist lol.
Martin