On a recent trip, the Eurostar ticket to Brussels (which is also honored on the train from Brussels Midi station to the airport), purchased in advance, was about GBP40. I believe the APD from London to the US is GBP65 in coach and GBP130 in all other classes, including many premium coach seats. So the savings is either GBP25 or 90, plus (in my case) the cost of the Heathrow Express, another GBP18.

The actual fare from BRU was also a little cheaper than the fare from LHR. I believe it may be because airport fees, which are in addition to the APD, are less expensive at BRU.