In that case, Alien is a good one (British director as well).
Others, more traditionally regarded as British:
2001
Dr Strangelove
Many of the Will Hay ones - particularly Oh Mr Porter, Ask a Policeman, My Learned Friend and Where's That Fire?
Britannia Hospital
P'Tang Yang Kipperbang
Unman, Wittering and Zigo
The Ladykillers
The Third Man
A Shot in the Dark
Local Hero
Gregory's Girl
That Sinking Feeling
The Cook The Thief His Wife and Her Lover
A Zed and Two Noughts
The Draftsman's Contract
Drowning by Numbers
The Falls
The Bed Sitting Room
The Running, Jumping and Standing Still Film
A Room for Romeo Brass
Rita, Sue and Bob Too
Alfie
Human Traffic
Get Carter
The Italian Job
The Long Good Friday
Lock Stock and Two Smoking Barrels
Final Cut / Love Honour and Obey
Train Spotting
Heaven's Above
I'm Alright Jack
The Wrong Box
Jabberwocky
Monty Python and the Holy Grail
Life of Brian
Time Bandits
Yellow Submarine
The Magic Christian
Johnny English
Hot Fuzz
Shaun of the Dead
Night of the Demon
Quatermas and the Pit
Peeping Tom
Dead of Night
Magic
The Wicker Man
Straw Dogs
The Elephant Man
Bedazzled
Carry On - Up the Khyber / Screaming / Abroad / Behind / At Your Convenience / Doctor / Again Doctor, etc, etc.
Casino Royale
Whistle Down the Wind
(OP - I picked up Letter to Breznev from a charity shop the other day but haven't watched it yet).