Hey guys. Fascinating thread - some great pictures and analysis. As someone who enjoys flying and plane watching, Flightradar24 has quickly become one of the most utilised apps on my phone! Like many posters on here I've always wondered 'just how far is that plane?' Thankfully now we know! With my app to hand and a watchful eye on the sky, below is a collection of photos that I've taken of planes at long distance with contrails. All photos were taken on an iPhone 6.
On some photos you may need to zoom or tilt the screen slightly to get a better view of the contrail.

Taken from the A110 near Oakwood (north London) overlooking Trent Park. The camera is facing west. Unfortunately I've misplaced the Flightradar screenshot but the hook of the contrail is over Gloucestershire.
The contrail (circled) is over Llandovery in the Brecon Beacons which is approximately 160 miles away. This photo was taken at 19:37 on Monday 15 August 2016. If anyone can track down further info that would be great.
Taken from Oakleigh Park Station (north London) at 09:03 on Friday 25 November. The plane was passing over Sandwich Bay which is approximately 72 miles away. I tracked the plane for another 10 miles until it dropped behind the tree line.
Flightradar info for the plane passing over Sandwich Bay.

Taken from the railway bridge at New Barnet Station at 09:33 on Friday 25 November 2016. The camera is facing east/south east. The visibility was phenomenal. Interestingly, Barnet is on high ground and there isn't anything much higher between Barnet and the Urals on the Eastern borders of Russia. The contrail of the plane (circled) was passing over Dunkirk in northern France which is approximately 118 miles away. I managed to track this flight for another 30 miles but the camera was simply not good enough.
Flightradar info for the flight passing over Dunkirk.
Nice and easy. Taken from Putney Bridge Underground Station on a crisp Autumn morning. The furthest plane is approx 37 miles away.
Both planes are on exactly the same flight path.