Measure your finger - there are a number of ways that you can do this.
Measure your finger at the end of the day. Do not measure your finger when it is hot or cold.
Remember that the two hands differ slightly in size; a ring worn on a finger of the left hand will not be a perfect fit for the same finger on the right hand. Make sure that the chosen size leaves enough room for the ring to pass easily along the entire length of the finger.
If you have an existing ring that fits the finger you wish to wear your Voyager ring on, you can measure the inside diameter of the ring in millimetres and using the GIA's International Ring Size Chart below, select the corresponding UK ring size.