Agreed, it's certainly not a pleasant experience going through Gatwick these days. The South Terminal is looking very tired now with filthy carpets, broken doors, plenty of black and yellow sticky tape and pidgeon droppings on floors and handrails......and that's just the journey from the car parks - a "first and last impression" !
In my opinion there are a number of factors in all of this :
1. The terminal buildings are virtually at their capacity, allowing no window for proper maintenance, cleaning and development work without creating further disruption and inconvinience.
It's all well and good talking about bigger aircraft and load factors to bump up the throughput on the runway(s), but the terminals can't cope.
2. Money. Of the other airports named, how many are privately owned and demanding of a significant return on investment ? BAAs new owners are in this category, and like all businesses in the UK consider cleaning to be an overhead to be squeezed, many often specifying reduced frequencies and not cleaning above "reachable height" as ways to contain costs.
3. Culture (1). Let's face it, in the UK we think of cleaning and cleaners as some form of underclass, and cannot recognise the value of this service until it is missing. Cleaners generally have the lowest wages and unsociable hours in society. OK, I know the work is labour intensive and does not require a lot of mental agility, but perhaps if they had better pay and conditions they would have some pride in the job which would reflect in the results.
4. Culture (2). Just whos mess is it the cleaners are picking up ? Not theirs that's for sure, so perhaps the travelling public, to some extent, get the environment they create ? It's the travelling public that drop the litter, spit out chewing gum, appear not to be toilet trained, use seats as footstools, spill drinks, leave empty cartons and bottles on tables, etc etc.