"I just had a look at the ticket prices, all examples I tried were around 30-50% cheaper than the equivalent BA seat. "
Which just goes to show they're having absolutely no impact on BA as otherwise they would be matching Silverjet's fares.
So as a consumer you have the choice: up to LTN for one of two Silverjet departures to Newark or across to LHR and choose one of 8 (I think) BA departures to JFK and earn miles and be safe in the knowledge that if the a/c goes t*ts up there's a better chance of back up from BA.
Such is competition: enjoy it while you can!