Not answering that question as such, but just a comment: If you get the booster free as part of your package, by all means try it. Sometimes they work fine, sometimes they don't, depending on the property.
If you don't get it as part of your package and have to pay the £3 monthly charge then they are outstandingly poor value, and you might want to consider alternative approaches. For perhaps £40 you could buy your own powerline wifi booster outright, from a reputable brand like TP-Link (although powerline networking always a sometimes-works technology). Ffity quid buys you the pretty decent Asus RT-AC57U router, and for £70 you could buy a cheap mesh wifi system like the Tenda Nova MW3.
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