Well as I say, the legislation is pretty clear that whilst goods are a "change your mind and send back in 14 days", services are "change your mind within 14 days of order before service provided". It is not a free trial.
The wording is messy, but they can argue you agreed to install within 14 days and they fully installed the service within 14 days, maybe, so you are no longer covered by the 14 days cooling off period.
I mean, best of luck, but this is a messy area of law.
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^RevK