There's plenty of threats by Putin to use first strike nuclear weapons if NATO troops directly engage Russian troops. So the current approach is indirect: British "military advisers" train Ukranians in the use of British Javelin missiles, but don't pull the trigger themselves.
Even a non-nuclear direct conventional war would get very messy very quickly. NATO would have to seize Kaliningrad and Belarus, start interning Russian nationals, rush forces to hold the Baltic states, and so on. It's not complicated to say "this will be very expensive and deadly, so we're not going to do it unless absolutely forced to".
European states are now re-arming (the irony of fate is that it's now US policy for Germany and Japan to re-arm), but that is going to take a while.
Even a non-nuclear direct conventional war would get very messy very quickly. NATO would have to seize Kaliningrad and Belarus, start interning Russian nationals, rush forces to hold the Baltic states, and so on. It's not complicated to say "this will be very expensive and deadly, so we're not going to do it unless absolutely forced to".
European states are now re-arming (the irony of fate is that it's now US policy for Germany and Japan to re-arm), but that is going to take a while.