Fire detection requirements

Other

I am getting a property ready for its first rental. It currently has no fire detection/alarm system. I have always understood a system of hard-wired detectors are required. However, this may be difficult to achieve without major upheaval. Does the RLA (or the law/advice) have a view on the installation of wireless detector systems? I believe that if one detector goes off, then all will be activated, but this may not satisfy any legislation regarding them being hard-wired, presumably they will still depend on battery power.