How are they any different from other charges - car parking, bags, etc?
1999 you would have strongly disagreed with 2019 you on this I bet.
The modern airline business conditions you to accept what was once abhorrent as the new norm, times change.
1999 you would be outraged at paying for seats for example, or flying long haul with no food.
Why not charge for security or access to the terminal building? Anyone who doesn't pay can queue outside for basic security entrance at a side door, all to the same standard as DfT but the experience not so good cos you didn't pony up.
In terms of customer experience, the scores have plummeted if you lack status and don't see these charges outright. If you fly from one of the more marginal UK airports, they REALLY need the revenue stream so they'll milk you for every penny, otherwise they'd do a Blackpool. As for LHR's drop off being free, that's likely to go in part for expansion as the green lobby will be thrown a sop.