I've never used a britrail pass before but it sounds so convenient and easy. Do you ever need to make a reservation or do you just hope that seats are available.>

Never need to do anything but hop on the train and in first class there are always lots and lots of empty seats IME - check restrictions with the Senior Railpass - I do not know - maybe it can be used on any train anytime or maybe there are black out periods? Again I do not know but if you want unfettered hop on any train anytime - no need to go to ticket window the pass is that but you have to be taking several train trips to make it pay off.

and time of year matters - Nov-Feb all passes are 20% off - and if you have more than two folks the third and on pays only 50% of what the first two pay - and kids under 16 get a free pass - those are variables that affect the efficacy of a pass.

I think since you are doing only a few trips you are best off without one.