The plan is to provide air cover for Ukrainian troops plus missiles.
How?
You probably know that the Russians have rather good anti aircraft systems, which have kept Ukrainian air power at a long distance from most of the battle zones.
What Starmer's stupid posturing does (aside from making him look ridiculous - because the Russians well know our impotent state) is to risk war between the UK and Russia.
It is one thing to supply help and advice in the form of intel and hardware. It is quite another to put UK troops into battle with Russians . As a lad I used to wonder how it was that British politicians were stupid enough and blinkered enough to stumble into World War 1 when we never needed to have that debacle. Likewise I wondered how it was that when Nazi Germany was building a massive war machine in the mid 1930s we didn't see what was coming and prepare. I look at Starmer and see him making the first mistake by proposing troop deployments that will bring us into direct conflict with Russia. Maybe he has time to avoid the second of those mistakes by cranking up arms production and manpower recruitment to unprecedented levels.
In the military build up part, he will find massive opposition from the left in his party. All they ever want is more money to throw at the black hole of the NHS and welfare. They are like ostriches burying their heads when a predator is stalking them.
Russia under Putin is intent on re-establishing control all around its current borders. The Poles, the Fins, the Baltics and the other front line border states know that very well and they only recently escaped from the suffocating occupations of the mid twentieth century when they were completely under Russian domination. Poland is spending 4.1% of gdp on arms. The UK spends 2.5% and much of it has been spent badly in the past. Germany is spending 1.5%.
Ukraine is spending 37% of gdp to keep the Russians from over-running them completely.
In the end, it is much cheaper to spend to deter war, rather than to have to fight one.