That's assuming UK authorities can even identify who is operating the Matrix instance. At the very least this assumes that a warrant is served to the registrar and/or the owner of the server/VPS in the correct jurisdiction, and that obfuscation measures were not taken by the operator. All of this will probably go nowhere.
I'm sure your server will be fine. Now if you put up a big declaration like that on your website, some bureaucrat might just decide to pick on you when they get the chance like France with Durov.