I've lived in the UK all my life, use trains quite a lot and have never come across or heard of any general 'summer timetable' release date. Train timetables in the UK (England, Scotland, Wales and Northern Ireland really don't vary very much across the year. The only real exceptions are the very few services on very specific branch lines which only run during the summer months. Otherwise you're just talking about variations of a few minutes or, occasionally, more services on a particular route. If you use the official railway website below you'll be able to put in a specific date and see train times from A to B on dates until June 10th (at the time of writing this). Services and times after that date won't change much, if at all. http://www.nationalrail.co.uk/
Elizabeth Thanks! I am trying to plan post June, so good to know the service times don't change much after that.
Mary I guessed that was what was happening! The website 'end date' is for booking online (and to allow for potential fare rises), not for timetables, so that's why it's there. The end date changes as the date changes. This time next week it'll probably be around 17th June and so on..... :-)