but c'mon, the British weather sucks--how long before these little "microlight" planes get knocked out of the sky by wind, attacked by birds, or grounded due to foul weather?

Actually, theoretically never. Not to give you an RTFA, but the craft hovers at an altitude of about 12 miles -- well into the stratosphere. The stratosphere begins at about 15km (9mi) from the Earth's surface and is composed of less dense, relatively stable air. I say relatively because there is a lot of lateral mixing but nothing quite as turbulent as what we experience on the surface. The highest clouds form not far from the tropopause (cirrus, stratocumulus, et al), so weather and harsh winds would have no effect on the craft at all.