The online advanced one way fares on HEX which have been around for a few years now are actually pretty decent prices:
Express Class - 14 Day Advance Purchase (Mon-Fri) £16.50
Express Class - 14 Day Advance Purchase (Weekends) £12.10
Business Class - 14 Day Advance Purchase (Weekend) £16.50
Express Class - 30 Day Advance Purchase (Mon-Fri) £14.30
Express Class - 30 Day Advance Purchase (Weekends) £8.80
Business Class - 30 Day Advance Purchase (Weekend) £12.00
Express Class - 90 Day Advance Purchase (Mon-Fri) £12.10
Express Class - 90 Day Advance Purchase (Weekends) £5.50
Business Class - 90 Day Advance Purchase (Weekend) £7.50
https://www.londontoolkit.com/travel/lhr_rail.htm