Back to the original question. Currently in the UK the rule is + or - 30 minutes.
There is a consultation in progress to change the rules of the air to a European standard in December 2012 (European Standardised Rules of the Air or SERA). The draft rules were written by Eurocontrol and attempt to get all of Europe onto ICAO standards with few differences filed. The UK filed a difference on the start and end of night.
It is proposed that all Europe use the start and end of civil twilight as explained in an earlier post.
Does it matter?
No. For the UK in the worst case it makes no difference. In the North of Scotland, in summer, civil twilight is up to an hour before sunrise and an hour after sunset so a much longer legal day.