I agree with the previous posters, it is an estimate that is often conservative. However, be warned that there are
rare cases where people get much less than the estimate. Even where they have estimated nearly 80, BT only assure you 12.
Also, don't forget that there are many others who will offer the same FTTC service as well as BT retail's Infinity. They all use the same underlying BT Openreach FTTC infrastructure, with their own internet infrastructure.
BT advertising is rather misleading in a couple of ways. They imply without saying it that Infinity is a unique product. They say that their router is really wonderful at wireless connection, where it is really an up to date but low end product adequate for many but very far from many better products.
Sky, PlusNet (a BT owned company), John Lewis (who use PlusNet infrastructure) are the commonest at the consumer level. Many others (Zen, etc) offer higher levels of service (though probably the same FTTC raw speeds) at much higher prices. See
https://spreadsheets0.google.com/pub?hl=en&hl=en&key.... That said, for me and for many who don't need fast p2p, BT Infinity ends up as the most cost effective choice.
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Moved (with trepidation) to BT Infinity 2 for upload speed. Happy BE user for several years.