If you have at least 7 days

You could order a cake from M&S or Waitrose (many can be personalised with a message).  There are even a few cakes that can be ordered with just 4 or 5 days notice.  Here are a few that appeal to me and that could easily be 'blinged up' by a novice cake decorator:

If you have less than 7 days

You could buy a 'ready to decorate cake' from a local supermarket and decorate it with purchased decorations. Cake decorations are available in supermarkets & from Hobbycraft. Our local independent cake shops offer the best service of all!

These two local shops are treasure troves and the staff are always incredibly helpful.  Just remember that Marilyn's Cakes is closed on Mondays.  Cakes O'Licious has recently moved and their website has not been updated.  The link takes you to their Facebook page which has more info.