You may have been confused by the fact that the FA Cup Final is staged at wembley along with the league cup and the playoffs and charity shield.
they involve league teams, the charity shiled often has man united in it(for the last 4 years).
Arsenal also played there two seasons ago for their european cup matches, prehaps that is wear your confusion lies.
Mancester United shirts are sold throughout the country in sprot shops a long with many other major teams, liverpool, arsenal etc.
A section of fans in london 'support' man utd, there are known as glory hunters.they have never watched man utd play and forsake great london teams like arsenal, chelsea, westham and spurs to support man utd.
i regret to inform you that wembley is no where near the houses of parliament, its in the suburbs of london, at the moment its closed and is meant to be rebuilt by 2004, though with this government famous for the dome as well, i dont expect much will happen.
krgds, ps did u like london?
It is amazing what can be accomplised when nobody takes the credit