I finally got through to the BFI IMAX website only to find out that all the good tickets for the Friday have gone. I had a look at the Birmingham's IMAX screen (which is now a Giant Screen, seems to be just a different licensing) which is close to me at home the week after and where I saw Avatar and it looks like I should be able to get tickets to that without much trouble (they are released two weeks before they go on sale). The really insane thing that blew my mind was the price - in London peak tickets are £16.75 and £12 for students, whereas in Birmingham they're £8.50 and £6 (plus a cheaper booking fee).
If that should fall through I'll just get some tickets to one of the Leicester Square cinemas. I saw a lot of big releases at the Odeon there over the years, and it's got terrific atmosphere when it's full (Return of the King was given a standing ovation on opening night). I only recently went to the Empire Apollo there and was really impressed by it (although there was only about 20 people in the screening) so I'd quite like to try that with a big crowd too.