Paulh53, regardless of the regs (building or electrical), I agree with Breezer that fitting mains powered smoke alarms is always a good idea.
My father is an ex fire fighter, and my father-in-law is a serving fire fighter. I have therefor heard many stories of people found after fires, unburned but dead from smoke inhalation. Working in the property rental market for a while, I know how many people (lots) take the batteries out to stop alarms sounding whilst cooking, and then forget to put them back.
Fitting mains smoke alarms as part of a rewire is not much more time consuming than a few light fittings, and the extra cost (at £12 ea alarm trade + labour + vat) is far less expensive than a funeral. If you are fitting them retrospectively , then linking them together may require running a 3P + E between the first floor and upper floors, which will require tidying in some manner.
I always fit them into any rewire. Round here a large number of houses are rented, and some of the better agencies insist on main powered smoke alarms. Everyone else we tell the kids in the wardrobe stories (this apparently is where most kids hide from fires and choke to death), to get the point across. Whilst landlords are notorious for cost cutting, dead tenants don't pay any rent.
Put a detector on each level of the property (ie basement, ground floor, first floor, second floor, loft conversion etc.), wire them all off of the same supply, and connect the links. Up to 12 can be linked. This way if there is smoke on one floor, the alarms on all of the others will sound too. This also comes in handy to summon everyone to dinner, as my cooking will usually trip the detectors! In a domestic situation, best not to put a detector in the kitchen as the heat from cooking will continuously set it off.
I usually feed from lighting circuits as it is rare that even really stupid people will leave the power to the lights off just to shut up an alarm tripped by cooking.
BTW You can get mains powered detectors for heat, smoke and various gases / ions too, as well as combination units, so check that you are getting the right type. These are usually used in commercial settings, where heat may be expected but not smoke (kitchen), or vice versa.