I think the short notice is about right now. The practical matters, erm, matter a lot (you can tell I'm an English teacher), not just because SMT might be out, but because half the school might be out on a trip. Also, some schools have 'project weeks' when you can't really judge what the year-round teaching's like, and individual teachers might have prepared something unusual and not representative (which could be good for them, perhaps, but not good for a realistic inspection). Additonally, it's because Ofsted, those incredibly efficient people, just might not take notice of local school holidays or inset days and the like, and turn up when the school's closed.
Besides, it's like a job interview in lots of ways - for your own job - and you'd expect a little preparation for your job interview. You wouldn't expect to be dragged in for an interview on a day when you're fluey, you're wearing clothes that you had to sniff to check if they were OK, and the printer's eaten your CV. There's nothing serious you can really cover up in the short time allowed. With no notice at all, too much would depend on luck.
We're due an inspection this year, though, and I'm actually looking forward to it. Not that I'm so wonderful - just that I've been so incredibly relaxed this year and winged it in every single lesson and lesson observation, and they've all gone really well, so I'm hoping my luck will hold.
*sends some of this luck towards spangles and milly molly*
*keeps some back for self anyway*