It's a dying industry and you would be better off as a 'spark plus' e.g. be able to also competently do fire / access / networking - a nice little earner on small commercial type works.
Im my experience of both...
People wont even question you for a spark quote if it's something 'nice' e.g. a conservatory, extension, man cave etc... or they are desperate to get an office finished for a business. Especially if the builder is wanting £50k, your quote usually just 'disappears'.
People will fight over the last £5 or 'I might just get my mate to do this, thanks for coming' over an alarm
(though to my utter amazement mid-priced, non Barrat housing estate round the corner was hard-wired for alarms, with no apparent company on board with the builder...)