A good starting point is Early Childhood Transitions Research: a Review of Concepts, Theory and Practice; this is a 2008 publication from the Bernard van Leer Foundation which gathers together much of the literature already published on this topic.
The previous government carried out plenty of research on various transition types. Here's a list of them with links to the relevant reports, though bear in mind that if any official policies are stated, then they may not necessarily be the policy of the current government:
- Transfer and Transitions in the Middle Years of Schooling (7-14): Continuities and Discontinuities in Learning
Finally, the Teaching Expertise site has plenty of articles which are filled with practical tips on managing various transitions within education. There are far too many to list them all here, but if you do a search for 'transition' in the site's search box then you'll find lots of reading to keep you busy for a while.