The UK sizing system for rings corresponds to a circumference measurement given to a letter. For example letter F to Z.
Larger sizes are also available, for example Z+1 – Z+5.
A handmade ring will be made to fit your finger perfectly, so that may involve a half or quarter size included in the letter measurement, for example M and a half.
Ring size conversion charts are a great way to measure your finger and get a good estimate. If you are commissioning a ring, I will size your finger accurately to save any disappointment regarding fit. If you live far away, ring sizes are possible to post out to you, please enquire.