On 19th December 1994 the UK and the US, together with the Russian Federation and the Ukraine, signed the Bucharest Memorandum; pledging to respect the independence and the borders of the Ukraine; to refrain from the threat or use of force against the Ukraine; to refrain from economic coercion and to seek immediate UN Security Council action in the event that Ukraine become a victim of an act of aggression.
So, there is no explicit or even implied requirement for the UK or the US to run to Ukraine's aid other than to go to the UNSC, which they did and which unsurprisingly was vetoed.
However, it does mention 'threat' of force and I reckon the threat is manifest, regardless of whether any triggers have been pulled.
Russia, or more accurately, Putin, has now blatantly broken these pledges. So London and Washington could say they have fulfilled their obligations by going to the Security Council and leave it at that, but they did sign the document, it does link them to the Ukraine.
United Nations Official Document
I have argued, albeit not in this forum, against defence cuts for decades and I despair for the security of the 'West.'
For decades the people elected to protect the Realm have wreaked grievous harm instead, usually citing money as a cause.
Often, far to often, they have failed in their duty, but those who live in the West and cast Putin as a role model, who argue in his favour, such people I would cheerfully send to a Siberian holiday camp.