Fun and games of regulated asset bases, me thinks. There was a rumour (probably a myth) that the bill the regulator saw for a tree to be removed in a car park was over £30k.
That said, £1.8bn sounds high, when the expansion project, pre supply chain crisis and associated inflation, was in for around £15bn (lets not have a debate about whether that number was realistic or not!). But there is plenty of aging infrastructure in that part of the airport, so I expect its far more than just the baggage system. The tunnels connecting to the M4 were, for example, in a sorry state 2 years ago when I last had discussions with the civils team at Heathrow...