I'm afraid you're mistaken on that front. Marylebone's at 205 miles 77 chains from Manchester.
The 241mp is on the Schoolhill side of Aberdeen station. You’re correct though, it is the highest milepost in Scotland.
Is that because the pre-grouping railway companies had different starting points, such that, for instance although it must be about 270 miles from Stanley Junction to Wick, the Highland Railway measured all their distances from Inverness?
The Quail (mine isn't the latest version) shows "Wolverton deviation 51.40-52.72 (35 yards longer)" but no change of mileage is shown in the vicinity. As usual practice is inconsistent, as the re-alignment at Newton Hall north of Durham shows a change of mileage from 67.71 1/2 to 68. Where the mileage changes there should be a change of Engineers Line Reference but here there isn't.
Děčin is even more interesting. It's actually two towns, with the old Děčin on the right bank of the Elbe/Laba and the old Podmokly on the left bank, represented in the last Sudetenland days as Tetschen-Bodenbach. The main railway route is that on the left bank, and the kilometres from Vienna via Prague end in the main station at 539.683. This is also the point where the Saxon State Railways line on to Dresden has its zero-point, well on the Austrian (Bohemian) side of the historic border between the Habsburg Empire and the Kingdom of Saxony. But there was another route from Vienna to Děčin, that of the Austrian Nordwestbahn, running on the right bank before crossing the river just north of Děčin to a junction with the Saxon line at what is now Děčin-Prostředni Žleb at 4.070 km on that line and 459.206 km from Wien Nordwestbahnhof. Again, the old measuring of the imperial railway carries on through what became Czechoslovakia at the end of the First World War—the Czechs, in marked contrast with the Poles, never bothered to change things! Heading south, the Nordwestbahn line does not diverge towards Prague by leaving the Elbe/Laba valley and following the Moldau/Vltava valley, but carries on broadly along the Elbe/Laba valley until it eventually turns away to the south-east and runs through Havličkúv Brod to Žnojmo (the historic boundary-town between Austria and Moravia) to the present international Šatov/Retz border at 87.7 km. In Vienna, the Nordwestbahnhof is still there as a freight depot, but no longer connected to its former main line because the Danube bridge is gone. The Nordwestbahn line is accessed over the Nordbhan bridge via Floridsdorf.
There always used to be two sets into York, with the GN mileage continuing on the east side of the line north of Chaloners Whin and the NER series starting from York on the west side. At some stage after the opening of the Selby Deviation they were changed, the NER posts being removed north of Colton Jn and the official measurement into York for all four lines being the GN sequence from King's Cross.
That line was not remiled.
London to Leicester was marked out with kilometre-posts many years ago for the APT-E trials. When those ended, the fairly flimsy posts were just left, and a few were still to be seen quite a few years later.
Converted to km, with all distances measured from Brussels Midi. Isn't that the primary reason the nation voted for Brexit?
Are Irish rail distances in Km or miles?
