Go to UKOpenMic.com

The "walk-in" Sign-up is usually 7 pm. However, a few nights have a 6 pm sign-up.

You can check the most up-to-date timings on our schedule page: http://ukopenmic.com/venues

N. B. It's worth noting that you can reserve your desired slot in advance on the above link, which is particularly advisable for busier nights, such as Saturday