In my local area, there are 3 regularly scheduled ECS moves, though they aren't as long as some already mentioned. The most regular is Lincoln to Cleethorpes, weekday mornings to bring the Sprinter for the morning Cleethorpes to Lincoln/Newark, it's also the shortest at just over 47 miles.
The next is to bring a fresh unit for the Cleethorpes-Barton line. This usually runs twice a week, depending on maintenance needs, Sheffield to Cleethorpes and depending on the route taken could be up to 72¼ miles.
Once a week, on a Saturday evening, a TPE 185 travels ECS from Cleethorpes to Crofton via Doncaster, which is roughly 65 miles.
Again, they're not as impressive distances as some moves already posted, but over 70 miles ECS still seems pretty crazy! Once upon a time, there were unusual in service workings at either end of the day to get trains in the right place. And of course, once upon a time many moons ago, there were more suitably placed depots, and ECS moves were minimal!