There are several ways to sever a joint tenancy, the most usual one being by making a unilateral declaration either orally or in writing. Thereafter, if there are two of you, the shares are held in a 50/50 joint tenancy unless you come to some other arrangement. Your local District Land Registry Office must be notified on their Form RX 1. There is no fee. Normally I would link you to the LR Form as it can be printed off their website, but unfortunately the LR site is down as I type, so when it re-appears just go to LR and Forms and you will find it by scrolling down.