Your insurance company is looking for some assurance that your alarm will do its job in the event of another burglary, and if it doesn t that they have some recourse, ie the alarm companies efficacy insurance.
So when they ask for alarm maintenance contract what they mean is they expect you to have a real alarm system, not something that came from Amazon and might alert you if you have another burglary. The Nest system is not compliant to any of the required british standards, its not graded. Even if you found some back street electrician that would offer a service contract, its unlikely that your insurer would pay out in the event of another successful burglary.