I have continued to use the giffgaff sim, the credit never expires, the alarm gets a test every now and again and it still has a whole loads of credit on it as it has done for the last year or so.
No top ups needed in 1 year, literally just 1 or 2 texts a month when I want to remotely set / unset, an AC failure happens, and when I test it.
I don't have a lot of faith in giffgaff and wouldn't use them as my main day to day phone as I know I arrange to meet a friend and then call when I'm nearby and we'll sync up, but the network going down would spoil the day, or work use it to call me so I need something really reliable.
However for an alarm giffgaff is good enough that you'd have to be so incredibly unfortunate to be burgled during one of their outages. Also to an extent if it was activated I could quickly confirm by checking the IP camera, but all I could do is call the police if I knew I was being burgled, in reality it may make no difference other than 'knowing' and limiting the amount of time they are likely to spend in the house. I guess if I was really really lucky the police might get on their tail if I confirmed a burglary in progress, but highly unlikely.
Still I'd rather know if somethings wrong and be able to rush back.