While I think that proper alarms are a good thing and should eventually be a regulation, this move has been poorly thought through and no provision I know of to support those unable to afford a professional system but unable to manage a DIY.
The weaselly worded cost estimate (based on cheaper alarms and DIY install) is misleading the public about its impact, as is the gov claims it won't be enforced, etc, but maybe insurance will start using it to deny payments, etc. Also the claims it is base on / to avoid another Grenfell Tower tragedy, even though that class of building and the serious aspect of inflammable cladding (still a WTF? for building standards) is utterly different to the vast majority of properties this applies to.
TL;DR Not thought through, should have been based on some aspect where it is managed as part of larger work anyway for non-rental property (e.g. on property sale or rewire job so managed over many years until feasible to force remainder to be upgraded)