Sven,
No, I haven't heard of them.
You need to follow a CAA-approved course before sitting the exams. The complete list of CAA-approved schools is
on their website, and I don't see a school called Horizon listed as offering any groundschool. The only Horizon in their list is Horizon Flying Club at Elmsett, and they seem to be an instructor school.
This might explain why Horizon.sfa are much cheaper. Of course it's possible they operate under another school's authorisation, or under the authority of a different JAR state - but some more research would definitely be needed before spending any of your hard-earned cash or time on anyone not on the official list.
FFF
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