Thank you for all the replies. I'm taking a note of every film you suggested and I have a great deal of films to find information about and look for now, as I don't know many of them.
Since I started this topic, some more films and filmmakers occured to me that would maybe have to be included:
How about a Ken Loach film? From my perspective, he is one of the most important British directors.
How about a "Carry on..." film? How important are these? I must admit I only got to know about them thanks to Total Film's 2004 list of Top 50 British films (which includes many of the films you allready suggested) and I think they are practically unknown in our country, so I really don't know about them.
How about a Norman Wisdom comedy?
And how about a couple of "contemporary" films? You know, not a classic that has already proven it can stand the test of time. You already mentioned some Kenneth Branagh Shakespeare adaptations. Which other film(s) from, say, 1990 onwards would you select? (recently I liked Hallam Foe very much, which unfortunately didn't get a broader distribution in other countries I think, not in ours at least).
I've never heard of It Happened Here, I think it was never released or shown on TV in our country. How is it connected with Czechoslovakia?
The Haunting is US film (I think), so that doesn't count.