Cheapest way of getting from LHR to London WITHOUT train or tube

I notice there will be a transport strike when I arrive in London next week, at the moment thinking the best way to get into central London may well be the car I have hired for 16 GBP (for two of us) picking up at LHR, returning at Edgware Road just outside the central toll zone. With 800 velocity points as well (Hertz rental) thrown into the mix (covers the fuel cost!), this seems sensible. Any other ways ? From what I can tell tube and Heathrow Connect will not be operating, and I think Heathrow Express is at reduced frequencies.

 

I had a 10% discount code for being a Singapore Airlines PPS member, but really that's not much. These rates were all through the hertz.com.au website, and only included manuals, automatics were 50GBP per day or more!! Have made a booking of a manual (at a similar rate of 16 GBP per day) in Edinburgh too.

And thanks, I found the National Express buses, and they seem they might take a while too in traffic and cost more. It seems now the Heathrow Express (42 GBP for two of us) will be running at 30 min intervals so it might be best, although more expensive. And the tube will be running to Acton Town, where you can change to the District line ... and another change or two further on (a pain in the proverbial).