Do NR have particular requirements as to max travel time for signallers? I can't find anything specific on the Careers section
Not that I'm aware. As long as you can get to work on time and your commute doesn't cause excessive fatigue, I don't think managers are especially bothered.
I've heard of signallers commuting daily to big London boxes from as far afield as the Isle of Wight and Lincolnshire. Wouldn't want to do it personally, but...
Alternatively, if the box works to a 12 hour roster (typically four on, four off) there is the option of staying locally for a sequence of shifts. One bloke at Ashford lived in Bristol because his other half didn't want to move. He used to drive up for each 'run' and slept in his van in the car park!