Generally, the police will not respond to alarms unless there's something that indicates there is a crime in progress. Which. of course is a very small minority of them. So if you have one where a notification is to a monitoring company, then them calling the police meets the same policy from the police. So they can, of course send one of their operatives. But don't expect them to do anything pro-active if something is actually happening, even should they get there any time soon. Except call the police. Who.....etc. etc.
An alarm activation that contacts the householder these days is the best alternative, price-wise as much as anything. As the householder, you can then go yourself or ask someone nearby you trust. That won't stop a burglary of course, but at least it'll mean there's someone to hand who can remedy the situation somewhat afterwards.
Relying on catching burglars really is bolting the stable door when the horse has fled. Security is key. Good doors, windows, lights, cameras, a barky dog or two and the place being occupied all help prevention. And it holds true that a determined burglar won't be stopped. Even lights and loud alarms will just put them on notice to hurry up. But even being in the house isn't always a total success. Remember George Digweed's harrowing experience?
As for whether you need an alarm, then there's nothing specific in law, but as above, the police will require one if you have more than a certain number of guns. And although you don't have to, then the police can then say that your guns won't be secure enough and not issue a certificate.
The over all policy to have as a certificate holder in my view is to do whatever you can to reasonably secure your firearms to the point where if the worst does happen, then when the police look at things after any (hopefully non-existent) burglary, they cannot fault you for being remiss in your security measures.