Im assuming you are catching a flight. On that basis Ill suggest AP fares going there but NOT coming back (in case of delay arriving).
Firstly- Bath to Hayes & Harlington (I assume you will be travelling with luggage, therefore I advise to add VIA Iver into the journey details, this simply produces the trains with least changes, i.e. instead of changing at Reading AND Slough, you just change at reading. It adds ten minutes or so but saves aggro of changing trains with luggage. The ticket costs the same) :
At 1713 on 31/3 is £10.55 (£14.85 FC) and gets to H&H at 1910 (depart Bath on the 1713 London Padd train arrives at Reading at 1811- Catch the London Paddington (stopper) departing at 1832 arrives Hayes at 1910).
If you don't mind being early, and want to save £££s get the 1613 arriving at H&H at 1810 (Bath 1613 - 1710 Reading .... 1732 Reading - 1810 Hayes) . This costs £8.90 Std.
Get a Anytime Return with YP H&H to Heathrow for £6.85 train leaves H&H at 1923 arrives at the Airport at 1935 (or 1823 - 1835).
Then get a Super off Peak single with YP from H&H to Bath Spa (NOT London) for 18.55, HOWEVER this only allows on the 1833 from H&H or later (which on the 1833 changing at Reading gets you to Bath at 2029)... Split if you want at Didcot IDK if its worth it !
Total price for that is £35.95 altogether (or £34.30 if you wan't to get the earlier train out of Bath on the outward journey, and get to the airport early).
That isn't bad I don't think....believe me its no fun going by bus or coach to airports, stick to rail services where possible. The worst part will be chaining at H&H over two flights of stairs. I believe at Reading there is a lift