Sylvester 5 day in Belgium (sometimes Holland) 26th Dec - 30th Dec, great fun. I went in 2007 and 2008. They have been good at giving the Brits late starts on the first days. Nice atmosphere, warm competition centres with showers/cafe/bar. Areas were fairly similar each day (reminded me of Berkshire) and not too far apart.
WRE middle distance event is this year held on 26th Dec.
Eurotunnel about 6am. Helps to live in the SE!! Ferry is a slower but often cheaper option. We arrived in good time for the last start slots.
We skipped the first day last year, choosing family time on Boxing Day instead - but this year it is the WRE on Boxing Day so will probably go. You can enter separate days so don't pay for what you miss.