I'm thinking about having a British film marathon

Since the Olympics are on I would put Chariots of Fire on your list.

 

Add these:

The Wicker Man (the original... duh )
Bronson
Children of Men
Lock stock and two smoking barrels
The 51st State

 

Edit : Deleted

 

That list needs a heavy dose of Harry Brown.

 

If I could only make one suggestion, it would be Joe Wright's Pride & Prejudice (2005). A modern classic, overlooked because of its subject matter, but it is really, really good. Great direction, smart script, perfectly paced and a great cast that all knock it out of the park.

Thinking about variety, other suggestions. I really recommend my top 5 in terms of variety. Also some of these would be considered Irish.

Pride & Prejudice (2005)
24 Hour Party People (2002)
Death at a Funeral (2007)
Brazil (1985)
Trainspotting (1996)
Love Actually (2003)
Bridge on the River Kwai (1957)
A Clockwork Orange (1971)
This is England (2006)
The Full Monty (1997)
Local Hero (1983)
The Long Good Friday (1980)
Hunger (2008)
Atonement (2007)
The Third Man (1949)
In The Name of the Father (1993)
The Guard (2011)

 

Hm, there's lots of classics. A few that just pop up in memory that hasn't been mentionend.

Nil B Mouth (1997, actually directed by Gary Oldman)
The Remains Of The Day (1993)
Monty Python And The Holy Grail (1975)

 

Edit : Deleted

 

Death at a Funeral
In the Loop
Tinker Tailor Soldier Spy