I don't know the exact figures, but the FTTC (VDSL2) signal degrades much more rapidly than ADSL2+ which is what the "Unlimited Broadband" is.
Your speed for 1300 metres to the cabinet plus from there to the exchange sounds fine. But once you go over 1000 metres from the cabinet
on FTTC things get very dodgy. Distance is the main determinant of speed, but things like the quality of the actual copper line from the cabinet to you are also important. If there are stretches of aluminium instead of copper then that really knocks it.
If you look at this graph
you get the idea. There are other graphs a bit different but not a lot.
But! The important thing affecting you is that BT (Consumer/Retail) will only offer Infinity if they can be pretty sure you will get 15Mbps or more on it. They do not offer Infinity if the estimated speed is below 15Mbps.
As far as I know all other ISPs do offer FTTC well below 15Mbps. Some as low as 5Mbps in the past though I don't know now. "Infinity" is just BT Retail's name for their over 15Mbps FTTC product.
If you put your phone number into this checker
, what do the VDSL A and B lines say?
BT do actually offer FTTC below 15Mbps, they just don't call it Infinity. IIRC it is something like "Faster broadband with fibre". But as I said, you may find Plusnet, Sky, TalkTalk and many smaller ISPs are happy to supply.
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