Cadbury World may be a long way for a not huge reward, but it can easily be done in a day.
If, as Emma say, the choo choo go that day.
From Euston Station, take Virgin West Coast for a fast trip or London Midland for a considerably slower but less expensive one, based on walk up tickets - both companies offer Advance Tickets at a much lower price but with a load of very specific restrictions - to Birmingham New Street, and change to the train to either Longbridge or Redditch. Just a few minutes later you will fetch up at Bournville. You can tell you are there by Cadbury purple everywhere. Cross under the tracks (don't go the wrong way or you may fall into the (very very cold in December) canal) into the station and step into Cadbury's.
Don't raise your expectations too high and you can be pleasantly surprised. Buy as many seconds and mis-shapes as you can carry.
Say hello to Brummagem on the way home. If you have spare time in Brum, a visit to the National Trust Back-to-Backs which show you very clearly what life was like on the first rung of the social ladder 150 years (until quite recently) ago. It is just a short walk from New Street Station.