Will be interesting to see how BA and Virgin will cope with the loss of their lucrative LHR to US oligopoly.
In reality, what difference will it make? LHR is completely slot restricted, "open skies" or not. If other carriers want to operate out of LHR to the USA, they'll have to buy space off someone else (if they can).
On the whole, I think it could be advantageous to Virgin, BA, Midland... as they should be able to fly from anywhere in the UK to the USA rather than being restricted to certain airport pairings. At the moment BA can't fly direct to IAH from LHR or ORD from LGW, etc. They could also take advantage of other European airports that are not at capacity.