Which border? Their western border with Poland, Slovakia, Hungary, Romania and Moldova? Their eastern border with Russia? The eastern border with the Donetsk and Lugansk separatist regions, either at the claimed or the de facto borders? How about the Crimea?
We can't (or won't) even control our own border with Mexico. How can we control the Ukranian-Russian border? And the British special forces would be better used defending their own Channel coast.