I'm not suggesting it would actually take place in the real world, but in a hypothetical scenario assuming their forces were positioned near the UK like Russia is around Ukraine, we would lose.
TBH if we lived in a situation where China could build up on our borders like Russia has done to Ukraine our military would be far more beefed up than it is now.
so they are not going full on, whilst Ukraine basically are throwing all viable men at it.
What kind of numbers are we talking if Russia decided to go all in?
Ukraine isn't throwing all available men at it though at a significantly heightened level of mobilisation - there are way more trying to sign up than they can use currently - only around 30% of their existing territorial forces are deployed to combat roles as well. As things stand it makes things a lot easier if they can minimise the number of inexperienced troops involved in combat, etc.
If Russia went full in, without significant mobilisation, they'd probably top out around 500-700K combat troops including air force, etc. not including support personnel. But if they went full mobilisation, mass conscription, etc. they can scale up well above that - probably around 20million or so though they'd struggle to support an army above 1-2 million fighting men in any real capacity without a few years of ramping up.
As it is with this invasion what were essentially conscripts being sent into combat in the early days were provided with a rubbish old AK47/74, 3 magazines of ammo, 72 hours of rations and that was basically it - things didn't improve much for them.
EDIT: I think people vastly underestimate Russia in this respect - albeit they don't have the same manpower and resources as the USSR, etc. but in WW2 they went from almost nothing with their army on the back foot to raising an additional 2.5 million men and equipment in around 5 months with a lot of that happening in just over a month. Albeit helped by allied supplies somewhat. But even today they can scale up to supporting a 70s era fighting force in large numbers quite quickly despite all the talk of sanctions and their woeful economy, and that will still be a significant force for Ukraine to deal with without more modern tanks and combat aircraft in numbers.