Indeed. The USA and rest of Europe would not just do nothing if Russia attacked the UK.
The UK is one of the most influential and powerful western nations/allies in NATO.
I think there is much less love for the UK within NATO than you think - we have power and influence, sadly being eroded by short term thinking, which grants some deference. I'm not so sure some countries would hurry to put themselves in the firing line for us if it came to it - even though that weakens NATO's deterrent value.