As a related question, if I replaced our existing wired smoke detectors with battery operated models, is it quite easy to “patch up” the wiring once the units have been removed? Ie. can the wires just end up in a chocolate block type terminator?

I have mains-operated smoke alarms in my house.

There's a 'signal' wire which carries the 'alarm-on' command to all others in the network.

I've doubled-up the use of this as an input to my Whole House Ventilation System.
When an alarm detects smoke, it kills the connection to the fans in the loft unit, thereby no longer blowing fresh air into rooms which may be alight. 😲 

This feature is not a mandatory requirement for domestic properties with MVHR, but I feel that it should be.

Yes @editor you can terminate wires if you replace a mains alarm with one that operates from battery. But screwed 'choc-blocks' are not permitted in ceiling voids any more.
You should be using a crimp/clamp style terminal, such as those made by Wago. They, in turn, should be held within an outer box.