You won’t actually commit a major criminal offence if you gain entry and disable the alarm.
It’s not Burglary – no element of intent to steal / cause GBH / commit criminal damage etc
It would initially be criminal damage, but you would have a statutory defence as you believe you would have consent if your neighbour knew of the circumstances of the damage.
It is being ‘found on enclosed premesis’ but same defence as above.
In your shoes, I’d call your local Police force and tell them of your intentions – you could even invite them to attend and witness the scenario. Then enter the house causing as little damage as possible, disable the alarms and leave. Film the whole thing in case any allegations are made. Secure the property afterwards, then look forward to a rather awkward conversation with your neighbour on thier return.
*NB – this all assumes cordial relations with neighbours. They might be a bit miffed if a fire starts and the house burns down because you turned the alarms off…