Shooting legal advice
You might well get a reduced premium but I would ask you to consider another factor.
If the premium is reduced because of the alarm and you forgot to set it when you went out, the insurers might say you had failed to comply with the terms of the policy and refuse to pay your claim in full if the house were broken into.
You need to weigh that possibility against the reduction in premium.
I know several people who have an alarm but, for that reason, don’t declare it.