Amended as I have found in the UK MATS Part 1 a table which gives a definitive answer which says the transition flight level will be FL70.
Google UK MATS Part 1 (CAP493) and open up the Adobe download, search for Appendix A (page 479) and there is a table showing this, can't find out how to shrink the link so will post if I find out!!
Link to the pdf download
here, just open the document and type in page 479 at the top.