I agree that the speed looks high for that length of standard (0.5mm) copper cable. My rule-of-thumb is that you can expect SFBB speeds (25Mbps+) at around 1300 metres, and that service can run out anywhere between 2km and 3km. Upstream usually dies first at longer distances.
On the other hand, if the cable were 0.9mm, it would behave like a line of half the distance, with 0.7mm cable somewhere between. There isn't much of the thicker stuff around, but who knows: you might be lucky.
Back in the past, someone (using Plusnet) got a working connection at over 4km, just. They are bound to have been on something thicker than 0.5mm cable.
The TBB guide
reckons on 15Mbps at 1500 metres. It seems a tad pessimistic to me, but they do say they include a lot of allowance for crosstalk (ie high takeup).
This website is more optimistic than TBB, and would believe it to be a reasonable speed: http://www.increasebroadbandspeed.co.uk/2013/chart-b...
If you want to be more sure about the prospects for leased line - at least sufficiently well before you buy the house - I'd suggest talking to someone who sells them. I usually use the Spitfire website
to look at product details & costs, but I've never used their services. Worth calling them?
What are your prospects for 4G?