That would not be fun. It could even turn nuclear.
But the logic of the UK/US position is to allow that as a very real possibility.
“As an alliance we must draw lines in the snow and be clear there are principles upon which we will not compromise,” Johnson said. “That includes the security of every Nato ally and the right of every European democracy to aspire tao Nato membership.”https://www.theguardian.com/world/2022/feb/09/uk-prepares-1000-troops-in-case-of-refugee-crisis-if-ukraine-invaded
(Notice ‘lines in the snow’ instead of ‘lines in the sand’. Even at this juncture he can’t help being an idiot.)
As Putin recently explained the problem is if Ukraine joins NATO someone in Kiev might decide to pick a fight with Russia over Crimea. Russia will of course defend Crimea (as part of Russia) and NATO would be obliged to support Ukraine. A similar problem arises in connection with South Ossetia and Abkhazia – Georgia’s breakaway regions which are supported by Russia. Russia’s concerns are not about “carving out spheres of influence”. They are practical, rational, concerns in everyone’s interest. They just want to avoid war.
As long as the UK/US axis is saying that Ukraine and Georgia can be admitted to NATO they are in effect laying the ground for a future Russia-NATO conflict. Boris Johnson may be parroting the usual stuff about ‘democracy’ but in effect he is calling for a Russia-NATO war over Crimea. Do they know what they are doing?
Of course; one can say that if Russia “gave back” Crimea and gave up support for the regions of South Ossetia and Abkhazia there would be no problem. Perhaps; though even then either of these countries is perhaps capable of inciting a conflict with Russia if they know they have NATO at their backs. But, in any event, that isn’t going to happen. Right or wrong this means that if the UK is willing to allow Ukraine into NATO it follows that the UK position is that they are willing to go to war with Russia over Crimea. Do the British people actually want this? I very much doubt it. It would be particularily odd since 80% of the population of Crimea voted (and confirmed in subsequent Western run polls) to rejoin Russia. So much for democracy. It would be another imperialist war.
Meanwhile Britain continues to be an embarrasment on the world stage. Instead of diplomacy a constant stream of threats and denunications. Even the patience of the Russian Foreign Ministry has been tested and they have been forced to say: “The British side must be clearly aware that without a clear change in the tone of the speeches of representatives of the British leadership, productive interaction is impossible either in solving bilateral problems or in settling international problems”.  Quite. Do Liz Truss and Boris Johnson and Ben Wallace really believe that you can issue threat after threat, insult after insult, and then pop over to Moscow and conduct useful diplomacy? Obviously not. It is just embarrassing.
Meanwhile the media docilely publishes fake news on behalf of the UK Foreign Office such as the idea that it is Russia delaying the implementation of the Minsk agreements.  This is provably not true. Once can simply compare the text of the Minsk 2 agreements  with recent statements by Ukranian Ministers to see that Kiev is openly refusing to implement Minsk 2.
People talk about the fog of war. The West is not yet at war with Russia but there is a very thick and dense cloud of fog hanging over matters. That fog is the irrational analysis-free narrative lines produced by the State Department (and echoed by the UK Foreign Office) and fed to the population by their partner media agencies (aka. the ‘free press’). It is hard to see how any dialogue can take place in this fog.
But in reality there is scope to calm matters down. The West could make its support for Ukraine contingent on Kiev immediately implementing the Minsk agreements which provide for autonomy for Donbass. There is scope for a new INF treaty. While NATO cannot publically agree to a Russian veto on Ukranian or Georgian membership it would be quite possible to add a clause that NATO would not support Ukraine in a war over Crimea or Georgia in a war over its breakaway Republics. (Unless of course it really is NATO’s position that they are ready to use force to take back Crimea?)