These are two very different extensions.
.co.uk is something of a "default" extension in the uk, (along with .com which seems to have a high market share in the UK also), it has alot of usage, prestige, history etc.
.us on the other hand is the country code of the US in a technical sense only, it is barely used and thus values are low. The country code of the US in reality is .com. The reason is .com took off in the US and became the default standard, .us was a local govnment extension, it missed the boat a long time ago and is very different to other extensions like .co.uk, .de, .se etc.