These are the cake makers and patisseries listed in London, the ones most likely to take on a tiered cake. General bakeries are left out of this list; they are on the cake makers in London page.
A baker will want to know the date, roughly how many guests, how many tiers, flavours, any allergies and where the cake has to be delivered. Put that in the form and it reaches up to three of them in one go. They reply to you directly by email.
Wedding cakes in London
195 places in London that make a wedding cake, out of 852 cake makers listed in the town. Send one request and up to three of them reply to you by email.