BS5839-1 is specific when it says weekly testing must be undertaken, ultimately if the regs are not followed you have to prove via risk assessment why you have deviated.
The rule i also think about is if there was a fire and someone died could you back up an investigation and court appearance saying you did all the checks required from the British standards?
As already noted by Paul a weekly test is a simple activation of a call point to confirm correct operation of the Fire Panel, i always think it better to do this with relays enabled so you are confirming fire doors close and any other ancillary equipment is doing what it should
Cost is not an option when it comes to life safety systems.
Hope that provides some more clarity