The daily mail has a picture of her visiting Westminster Abbey, which makes them come to the conclusion that she will get married there:
Kate Middleton has chosen Westminster Abbey for Royal wedding to Prince William | Mail Online
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Still, I do not think that we will see a royal turn up like in Sweden.
1) very little royals know William, he never visited any royal event, and neither does his grand mother.
2) he isn't the heir, but the number 2
3) it is a crisis, as a signal Buckingham Palace might not want to invite too many foreign royals
TBH, considering the disinterest that the prince and his grand mother have towards European royalty I would find it rather odd if all the other monarchs would turn up. Esp. the Swedish royals, whose highest English royal guests at any event were the duke and duchess of Gloucester (who are lovely people of course). I believe that off the record it was commented that it actually was considered a snub by the RF in Stockholm. So in the Swedish case sending an ambassador would be good enough, or a Bernadotte count at the very best.
Still, for many continental royals it is a good PR moment to show up, so probably every country will send at least the heir. Past royal events show that as soon the English celebrate the rest of royal europe turns up only too gladly. In any case, i don't think the delegations will be too large. In Sweden the Dutch RF for example came with 7 members, the Norwegians with 6, the Belgians with 8 etc. I think in this case only 2 members per royal family will show up, for the reasons I stated above (they do not know each other and Buckingham Palace doesn't want too many foreign royals around).