Wow didn't know that how come they sell 1st class tickets for?
Looking into this in a bit more detail, the fare you quote is a 1st Advance. Therefore, the train will be booked to be formed of either Class 185 both of which convey 1st class.
There is one individual seat on a 185 if you can get it, failing that you may have someone sat opposite/next to you but probably not (depending on the time you travel), but even if you don't have your own space you will have more space.
Having a reserved seat can't really be given as a "benefit" of a 1st Advance over a Standard Advance.
Also the fact that some turns are booked to be formed of Class 156 units isn't really of a concern to you, given that won't apply to your train. In the unlikely event of disruption it is possible that your train may not convey 1st class (either because it is declassified or it is a Class 156) in which case you would be entitled to a refund of the difference in fares, which I calculate to be £6.20 (based on the difference between the
1EF and
ZEF)