If you're talking about troops in sizeable numbers to fight a prepared and well-equipped foe like Russia, it usually takes months to prepare with support from allies.It seems pretty mental to me that we spend ungodly amounts of money on a military and aren’t ready to use it in any meaningful capacity. If the intelligence services are to be believed they knew this invasion was likely for months.
We have very few combat ready battalions bar from the JRRF: you're talking about five to eight thousands combat ready troops who we can fly in right now.
But where would they go? If they went to Poland, we've have to then transport them across the border, but with what? That's where preparing transport comes in.
When you move on to our mechanised infantry or our tanks, the ones that are ready are in Estonia. So we either leave them or have to get the rest prepared?
They'll need getting combat ready, transporting (that'll take a while!) and then getting into Russia through Poland, which is never going to happen quick enough.
All our Type 45 destroyers and the new Carrier Strike Group are in Portsmouth, so they'd have to get ready to sail, move to the Med and then prepare.
We'd need to mobile the logistics corps and whatnot too. The latter bit is the important bit - they probably did know, but we didn't want to escalate.
The US and EU are reluctant to act too, look at their poo-pooing of the SWIFT proposal.