I think the AIS site uses a lot of client-side Java. There is no need for it - its entire function, plus all the reference data it carries (e.g. the UK AIP) could be served in plain ordinary HTML.
It's not as bad as the Eurocontrol website though, which required a Java download of about 20MB last time I tried it. Under NT4 / IE6 it worked just once, with the PC crashing every time after the first time.