Mind
Mental health information and support across England and Wales. Their pages on anxiety and panic are solid.
mind.org.uk
No Panic
Specifically for panic, anxiety, phobias, and OCD. They run a helpline and recovery groups. Directly relevant if you have panic disorder.
nopanic.org.uk
Anxiety UK
Support for all anxiety disorders. They offer therapy access and a helpline.
anxietyuk.org.uk
Samaritans
24/7 listening support. Call 116 123, free from any phone. Not just for suicidal feelings - they're there if you're struggling to cope.
samaritans.org