Will ATA carnets be stamped by customs in Dover or is there a facility at Eurotunnel to do this?

Yes there are facilities in GB for stamping ATA Carnets on arrival at both Eurotunnel and the Port of Dover, details of the locations can be found at

gov.uk/guidance/moving-goods-through-the-port-of-dover-and-eurotunnel-with-an-ata-carnet