Hi, we are converting our basement into a living space and will obviously need a smoke detector/fire alarm down there. When we confirmed the requirements with Building Regs they have said it needs to be hard-wired and interconnected with alarms in the rest of the house. Our current alarms aren’t hard-wired and we have asked if we can hard-wire the one in the basement and use wireless ones in the rest of the house to interlink but they have said it’s against regulations. Obviously having alarms wired in around the house is going to be costly and disruptive, I’m just trying to find out if there is any way to avoid this? 
Thanks in advance

You can have radio interlinked mains supplied smoke alarms, I would ask your BCO why you can't, but I would suggest Aico brand.

Of course you can have wireless linked mains powered detectors these days - in fact they almost all are these days and few places manufacture wire linked domestic alarms these days.
Just install the wireless stuff & get a certificate off the electrician saying it's a Grade D2 LD1/2/3 (depending on where the detectors are) system, issue it to the BCO and they have to accept it as it matches the requirements in Approved Document B which most BCO follow as the standard