I don't see how this could be construed as being an error fare at all. My interpretation of this is that AC is sweating bullets with WS' newest J product to Canada/US from LGW and WS's fare sale that they're simply matching the prices WS is offering (i.e.
1600 GBP return fares from LGW to SFO). If anything I would book hotels and everything else on reliance that the fare is valid to prevent AC re-negging on their competitive response to WS.
My maxim has always been:
There's no such thing as an error fare! There is such a thing as competition and airlines feeling like the competition is breathing down their necks hard and for them to mount a response.
-James