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
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/