Re: Er...
The UK's WTO membership is under the umbrella of the EU, and coming out from under it to be an independent WTO member isn't as easy as it sounds, because a lot of things are agreed with other WTO members at EU level and those will need renegotiating. For perspective: there are issues arising from the 2004 expansion of the EU which still haven't been resolved... 12 years later.
http://www.ictsd.org/opinion/nothing-simple-about-uk-regaining-wto-status-post-brexit