Our women refuse to breed with us, obviously other ethnic groups with more compliant womenfolk will eventually replace us.
Not helped by those of us white guys that chose to move to a country with friendlier women. Obviously it's easier to flee than to fight.
As for overcrowding,
Not IMHO, but the big boys own all the land and won't let anyone build on it. The Crown, forestry commission, the church, National Trust, RSPB et al. once these guys get hold of it, it's permanently out of use for the general population.