Heathrow is building terminal four, a terminal which will be a mini-airport, nearly as big as 1-4 put together and handling as much traffic as the planned new airport for Paris.
Erm, terminal 5 darling.
Heathrow needs two, or at least one extra runway. A BAA proposition has been around for a while and would involve buying 800 houses. Very expensive but nessecary.
Airports like Manston and Southend need to increase short-haul passenger options, improve the airport and improve transport links with other airports (for transfers) and to the capital.
Gatwick is close to saturation. It COULD be possible to open the second runway (read: taxiway) for continual use, although there would be little point, as it is ILSless, and simultaneous ops aren't allowed.
In all seriousness, a new airport ISN'T required, if use is made of current airports. Heathrow, and the south, would benefit greatly from two extra runways. It'll NEVER happen though -- the local residents and noise/regualar pollution people will put a stop to that. Not to mention that the government has already ummed and arred for years and will continue to do so. Just one runway would be a big feat. Buying houses along North Perimeter road would be a big no-no and all time and cost involved would make any government wince.
Southend and Manston could be good airport. Manston can take south-east traffic whose passengers aren't specifically destined for the capital away from capital airports. Southend can easily become another Luton or Stanstead IF people were willing to invest.
Manchester is ok for the time being, although needs to get out of the charter-trap it's in now. Manchester is pretty large now, has similar runway capasity as Heathrow, and has great transport links. There doesn't need to be any new airport in the North (despite what the pro-Finningley crowd says).
Liverpool has expanded, and is easyJet's base.
Leeds/Bradford could do with expanding -- it's near two large cities and could easily be a low-co base, IF they're sensible and reduce landing fees.
Newcastle, Teeside -- fine as they are.
Your bone's got a little machine