Helicopter antics in London are pretty unlikely at the best of times, and right now we're tooled up for the Olympics too.

The police have the power to stop the car, but not search it. Do diplomatic vehicles have toilets?

Could they get a diplomatic charter bus?

They'd have to get across the pavement from the embassy into the bus. The British have every right to arrest him there (either for violating the conditions of bail or on grounds there is an outstanding European warrant for him).

The embassy doesn't have a paved space large enough for the first 5 feet of the bus to fit?

The "embassy" is not a compound, it's just an apartment in a normal building.