If you go by train LGW is as easy to get to as LHR.
If you use Avios for short haul your MAN-LHR flight will cost you a large chunk of Avios in addition to the Athens flight (you only get the connecting leg for free when you travel long haul).
Your analysis that you might as well fly a low cost carrier as try to book a short haul Avios reward from the regions is an accurate one.
Don't feel that you must use your companion voucher at all costs. Plenty of people let them lapse because there is better value to be had elsewhere.
It's only really on long haul or if London is your local airport that Avios work well.