The only reason stated publicly for the Thames Estuary airport espoused by Boris Johnson is competition with continental airports. It is interesting that two of the three major competitors Johnson cites are not capital cities, which suggests that if expansion of air links really is vital to the UK, then this expansion should surely take place in the industrial heartlands, either in Birmingham, Manchester or Leeds. These cities will be linked to London by HS2 on a timescale rather shorter than that for building an airport in the Thames.
There may be a short-term gain of construction jobs, but the long-term environmental loss is far greater. As the RSPB said in 2008: "airport construction in the estuary would be the most destructive development ever undertaken in the UK. It would seriously hamper all attempts to cut the UK’s greenhouse gas emissions." Because the estuary is an important wildlife resource, it is protected by UK and European law. Add the number of people in Kent and Essex who would take to the law to protect their quality of life, and one can foresee only lawyers benefiting if the prime minister pursues this.