Heidi may have been paying for £350 for her return, but some lucky passengers could have been paying around £50. That's the reality of the low cost pricing structure, some will get it cheap and others expensive.
It some cases BA and bmi may be cheaper than the likes of easyJet, but it really depends on individual cases. The key message that BA should be getting across is that easyJet are not always the cheapest, so it pays to check around and not just assume that because it's a "low cost" airline that their fare will always be the cheapest.
In Heidi's case it would be interesting to know when she saw that easyJet was £350 whether she checked what BA were offering at the same time.