Instead of going it alone, spending much more than you need to and fecking it up look at how others do it!
Oslo, Stockholm, Riga, and many more. Situate the airport out of town, say 40kms or so and build a good transport infastructure to service it. From all over the country. Not a stupid expensive train to Paddington (which is not the centre of the city and everyone then has to get additional transport) where you have to walk 1/2 way to London just to find the train!
It's been done countless times already but will they follow a system that works? Not on your life. Just like the personel ID cards fiasco. It exists elsewhere so if you are hell bent on having the system, adopt a system with a proven track history not spend billions trying to re-invent the wheel only to find once you have done it you forgot to put spokes in it!
But only in UK do you have to live with this bo77ocks, no-where else!
'Having a bad day!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!