I fitted my (England) house with a linked system (probably 15yrs ago now), hardwired, various types of sensors in different areas according to the recommendations (ionisation, heat, optical). Hardwiring a circuit is a lot of faff unless doing a rewire or similar.
There are other options available now with wireless systems etc, which while expensive in hardware terms they can save an awful lot of wiring work so can end up cost effective.
My first units were mains/rechargeable back-up, but I replaced them with mains/replaceable battery types as being cheaper but also don't suffer from unpredictable rechargeable battery failures. Just fit a new PP9 every year or two when they start to beep.