Welcome to FT, skah9!There's a lot more help about this elsewhere on this board, but the first thing to note is that what you're referring to is probably not any form of Heathrow/airport surcharge, but a surcharge applied by BA. The consequence is that it seldom makes any sense to redeem Avios for long-haul economy flights on BA.

There are many other uses to which you can more sensibly put your Avios. Short-haul redemptions on BA are one way, particularly if you have collected any Avios in the last year and thus qualify for Reward Flight Saver pricing, in which BA charges only a small cash element on top of a small number of Avios. Or redeeming for travel on other airlines in circumstances in which BA does not charge surcharges of the same order, including short-haul redemptions on other airlines.

The saving that mikeyfly refers to is the UK's Air Passenger Duty, which doesn't apply to journeys which start in Inverness (even if they connect through Heathrow). But that's not the surcharge applied by BA. And I think it's only half that amount in economy anyway (actually £78 for a long-haul trip), so if economy travel is what you want to do, this much reduces the appeal of traipsing all the way to Inverness to start the trip. (It's possible that this makes up some of the £350 figure that you're referring to.)