The highest "West Coast" milestone is the 263 from Euston on the approach to Holyhead.
On the WCML proper, mileposts reset at Golborne Jn (187), Preston, Lancaster, and Carlisle, reflecting the original companies that built the line.
On the ECML, mileposts reset at Colton Jn (also 187), York, Newcastle and at the Scottish border north of Berwick (then counting down to Edinburgh).
In Scotland the highest milepost is probably the 241 approaching Aberdeen, on the Caledonian route from Carlisle via Perth and Forfar (some of which is now closed).
Foreign trivia: kilometre posts from Vienna increase all the way to something like 535 at Děčin in Czechia, near the German border.
Unusually, they have not been renumbered because of the changes in national borders 100 years ago.
They reflect the original posts established by the Nordbahn (to Brno) and the Staatsbahn (onwards via Prague), on what the Czechs call their "First Railway Corridor".
Km posts in the former Yugoslavia still have their origin in Belgrade.