You need to point out that you will be happy to accept Fater Broadband as you believe that is all your line will support and that you understand their reasons (marketing/sales/advertising) for not offering the Infinity product (in name only) for slower lines.
Then, say as you are accepting a lower standard of service, in terms of speed, then BT should be offering a lower price to compensate for that and that their costs will be lower as you will not be able to download anywhere near as much data. Aim for £15 month and settle for £20 would be my targets. Fairly sure I was offered around the £15 mark for a line I was working with.
If they will not move, then demand Infinity1 - point out it will work on your line but give sub-15Mbps speeds and that you are prepared to pay £xx for it.
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M H C
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