According to this source, a funding nation must be at 5 bars for at least 14 consecutive days by month-end before it will withdraw. Whilst I can't confirm this personally, I have played through at least one instance of a nation reaching month-end on 5 bars and not withdrawing. If the UK only just hit 5 bars then it seems you are in luck - you will get around 40 days to try and fix their panic level. If they hit 5 bars 3 days ago then you need to hope for a lucky council mission. In any case, I wouldn't advise rage-quitting before the month-end report.
More generally, keeping all nations in a Classic game is difficult. Even if you never fail a mission, on average the world gains more panic from the two abduction missions you miss than it loses from the missions you complete - this gives you a tight and somewhat inflexible time-scale to get all of your satellites up and requires that you build your engineer count and bank balance to the exclusion of all else. Satellite placement feels like making a move in chess. Your first priority must always be to defend against panic, rather than secure funding or continent bonuses, but play too defensively and you'll fail to generate engineers, uplinks or satellites quickly enough and simply be outpaced by rising panic. A no-withdrawal condition also leaves your soldiers relatively under-equipped compared to a lossy strategy, calling for greater acts of daring-do on the battlefield. It all calls for a kind of careful audacity and reckless caution.
I've completed the game on Classic Ironman a few times, twice with a full Council, and played several more games where I treated a nation's withdrawal as a game over. If you quit every time you lose a nation on Classic you will fail/abandon more games than you complete - even without failing any missions. A bad start or minor mistake will snowball quickly, and once a nation withdraws, there's no way back. Personally, I like the look of a perfect Situation Room screen, but if you play this way, it's a game you have to expect to lose.