Has anybody successfully activated a britrail pass at Heathrow?

I've done a bit more research on this, looking at the Heathrow Express website.

Although they are not a National Rail company they say that they do validate Britrail passes and that they are accepted by them.

From the "Railcards" link on their website:-

BritRail Passes

Holders of the following passes can use them on Heathrow Express,
providing they have first validated them at a Heathrow Express ticket
office or a main-line rail station.

BritRail Pass + Ireland BritRail

Consecutive Low Season BritRail

England Consecutive BritRail

Flexi Guest Pass

BritRail FlexiPass

BritRail Party Flexipass

BritRail Consective Eurail Passholder

Britrail Euro BritRail Point to Point (must state London-Heathrow and vice versa)

Passes marked UNDERGROUND cannot be used

Thanks Nigel !!!