If you can and want to imagine big war with Russia, before asking for more British Army divisions you'd better ask for ground based air and missile defence, because cruise missiles are the 1 threat sure to rain thick on UK, and there is next to nothing in place to deal with that.
As the UK is a critical base for fighting the movements of russian submarines, ships and long range aviation threatening the North Atlantic routes and entry point for troops arriving from CONUS, US would have to deploy lots of PATRIOT batteries to the UK just to keep it viable.
What else could the UK offer in such a big war against Russia before? Lossiemouth's P-8 Poseidons would be super busy. E-7? As well. Looking back, Lossiemouth used to have dedicate anti-ship attack jet squadrons to deal with russian surface movements, and that is no longer there.
Ark Royal IV spent the last of her service life cruising the North Sea and North Atlantic when NATO kept carrier strike groups there on an enduring basis to bring the fight UP to russian submarines and long range aviation, instead of waiting for them to get in "lossiemouth range"
In a big war against Russia, first things UK would end up doing would realistically be forward basing as many Typhoons as possible into Norway and the carrier group in northern waters to help close the air part of GIUK gap; and worry about cruise missiles still getting through.
Loading suggestions...