sounds too expensive. I pay less than that and I bet mine is better than yours.
get some more quotes. Look at the siren boxes on local shops and houses to see which companies are active, and phone them. Round here it's mostly SECOM, CIA and Chubb.
I have an idea that BG and some other utility companies have an alarms service.
Buy it, don't rent it. If you get a Galaxy or other major make, it can be maintained by another company if yours puts its prices up. Say that you will insist on knowing the Maintenance code for your alarm (so that if necessary someone else can maintain it and you are not held to ransom by the installer).
Ask what British Standard (if any) it meets and ask your insurance co which BS (if any) they require. While you're at it, ask them to email their Minimum Security Requirements to you, which will specify locks and things.
Although you might get a small insurance discount for an alarm, it will probably say they don't cover you if you are burgled and it has not been set, so I would be inclined not to tell them, to avoid the chance of having a claim rejected.