Some devices -- I'm not sure how many -- include the option to send a periodic test text.
With an alarm of any description, it's important that they send a test message regularly (probably once a week) to check that the service is working, has power etc. If an alarm doesn't have that capability, it's a serious design defect so you should probably be considering changing the alarm rather than getting a different account for the sim.
For less-essential devices, it's possible to keep them active remotely by adding credit every 6 months. But the minimum credit is £10 and a device that transmits infrequently (so as to need adding credit manually to keep it active) is probably unlikely to use more than a few pounds worth of credit in an average year.
The idea of having an account specifically for simple devices needing only small amounts of calls, data or texts spread over a long period is good. Probably a network could offer something like that for a single annual charge of about £10 per year. But I suspect it won't be from giffgaff, as giffgaff isn't currently interested in "small" users.