I think that each head of state should travel in separate cars. The bus is simply ridiculous in such a solemn occasion (it's not a wedding), and it may lead to diplomatic issues. I understand that the Italian president, among others, may be exempted from the bus. Why exactly? I'm pretty sure there aren't any relevant security issues about him (I'm Italian, in case somebody wonders). The bus-with-exemptions choice says "there are A-list and B-list countries".
I hope they rethink this arrangement: I know that a car for each one would mean an awful lot lot of cars to be driven up to the Abbey, but, come on, it can be done.