We decided to go with the Britrail pass for our journey (thanks for the recommendation there). I understand it's like a German train ticket (my only European experience) in that you can get on whatever train you want (for most trains, not the Caledonian Sleeper). However, my experience in Germany with 'unreserved' seats was mixed, particularly as we have two small children (and prefer four-together). We did buy a First Class pass, as it wasn't that much more.
Should we generally reserve seats in advance for medium-distance trips (2-3 hours), or is it usually possible at least in first class to get four seats together (I assume four seats around a table exists on most larger trains, at least seems to when I look around)? We did reserve a sleeper compartment on the Sleeper, as that was indicated to be mandatory. If we're careful not to leave on a commuting-type train during rush hour, can we get away with simply getting on the train and have a fairly high chance to get a seat together? We're mostly going to travel to the Birmingham area and Sussex (from a base in London, once we return from Scotland).