My SWAG is that RU will avoid an overt invasion, but continue with low-intensity and asymmetric pressure. Putin is trying to distract folks at home from their struggling economy... conflicts tends to do that, but the long-term ramifications of the inevitable western sanctions (more than what he has now) will only aggravate the situation.

Assume that Putin's big-picture motives WRT to eastern Europe are to retain those states in the RU sphere so as to assure a market for RU exports. Since RU cannot really compete with western innovation and efficiency, they need to ensure that their eastern market states stick with the post-soviet gangster paradigm of government. If other eastern states respond to a UK invasion by turning to the West (and worse if NATO embraces them), that would be counter-productive.

He probably doesn't want to kick that hornet's nest just to ostensibly "save" a handful of ethnic Russians from UK oppression