The UK Magic: The Gathering page on Wizards' social network has an announcement concerning the venue for GP London, happening Apr. 30 - May 1 which is the same weekend as the royal wedding. This guarantees that hotel space will be at a premium for the weekend so make your reservations as quickly as possible.

The event venue is:

Old Billingsgate

1 Old Billingsgate Walk

London

EC3R 6DX

Original announcement.