I was on at around 12:30 London time, and shortly after logging in noticed that the Exchange wasn't working anymore. Gave it up for a few hours and it's working fine for me now. It's quite responsive at the moment, which wasn't the case this morning.
I can't tell you I know why some people can access the site and others can't. I do know, however, that I have been unable to load the site and bet once or twice before, when others appeared to have no trouble at all. At other times, it was the other way around. I think UK-registered customers are redirected to a different server than non-UK-registered customers. There's a data server called uk-api.betfair.com; maybe there's one called int-api.betfair.com or something along those lines as well.
I was on at around 12:30 London time, and shortly after logging in noticed that the Exchange wasn't working anymore. Gave it up for a few hours and it's working fine for me now. It's quite responsive at the moment, which wasn't the case this morning.