Just did a bit of research....
15NOV NCL LHR BA1325 0950 1100
18NOV LHR NCL BA1338 1930 2035
Total price if booked today - £106.30 (including £5 discount if you take an eticket).
The 'low cost' alternative.
15NOV NCL STN GO614 0800 0900
18NOV STN NCL GO611 1850 1945
Total price if booked today - £77.40
So BA mainline is a grand total of £29 more expensive.
I can see for many people this would be worth paying. For a start, you get into LHR with its myriad of connections - and the HeX may be overpriced but it's the best airport rail link of them all.
Of them all, the train is the most surprising - over £160 if you want to take an early morning (arrive in KX by 1100) train.