The United Kingdom PLANS to leave the EU. It hasn't left yet, and could cancel even up until the very minute beforehand.
There are two scenarios - a 'soft' Brexit, in which case the UK is worse off but, because it leaves in an orderly fashion, not utterly wrecked - and a 'hard' Brexit (which is what we're almost certainly going to get) where absolutely nothing is decided and the UK crashes out with no deals in place. This likely lead to the breakup of the Union, with Northern Ireland possibly joining the Republic and Scotland voting to leave the UK, and who knows - maybe London and other large remain voting cities like Bristol becoming a city state.
There'll be mass unemployment - already many businesses are struggling due to the uncertainty and many internationals simply won't want their 'EU base' (as the UK is for so many businesses) outside the EU. Travel will be SEVERELY disrupted - it's possible that planes will be grounded as they simply won't be allowed to fly through EU airspace as there's (currently) no deal in place. Food shortages (the UK isn't able to feed itself on home grown produce), medicine shortages (including a near total cease of cancer treatment and x-rays), fuel shortages (the UK has nowhere near enough gas storage, everything's imported from the EU - and there is no deal in place) electricity shortages (due to nuclear power stations getting no fuel and there being no deal to import electricity) - you name it - ALL of this needs to be hashed out in *ONE YEAR*, and currently NOTHING has been sorted out. Not a single thing.