It's unlikely that a large wedding cake will conform to the regulations on dimensions for cabin baggage. You're only entitled, under EU security rules, to carry one item of cabin baggage and its dimensions must not exceed 56cm x 45cm x 25cm. Unless it's a single tier (or possibly 2 tier) cake, that 25cm limit might prove to be too restrictive.
(NB: Ryanair's own restrictions only allow cabin baggage up to 55cm x 40cm x 20cm. It seems even lesslikely that your cake will comply:
http://www.ryanair.com/site/EN/faqs.php?sect=b ag&quest=cabinbaggageallowance )
Additionally all 'liquids' (which includes creams and gels) have to be contained in a small clear bag. So, if the cake has soft icing, or layers of cream, it won't be allowed in the cabin.
You can view the EU rules here:
http://www.dft.gov.uk/transportforyou/airtrave l/airportsecurity/requirements
If, after reading the rules, you decide that you can comply with the requirements, you'll have to remember that the cake would form your sole item of cabin baggage. (i.e. you wouldn't be allowed to carry a handbag or any other items).
Unless you can find a reliable courier (who you'd really trust with your precious consignment), it would seem wisest to travel by ferry.
Chris