Rumours of Apple having issues producing and shipping enough new iPads to meet demand seem to be coming true. If you want to buy one online in Britain you'll now have to suffer through a 2-3 week wait, even if you pre-order it right now.
Normally, even if Apple's sold out online, you can pick one up in-store. You might have to queue, even over night in some places, but if you're desperate for one on launch day, or even anywhere remotely close to March 16th, that's going to be your only option.
Having done it once (for the iPhone 4), I can say queuing overnight at an Apple store is an experience. It's not one I wish to repeat anytime soon, but it wasn't wholly unpleasant -- there were quite a few people there with a fair amount of beer to see us through the night. Just watch out for the bums and drunks; they love pissing on sleeping, queuing geeks.