Yep, what they said! You need to get entry clearance to come into the country, so you get a visa and the visa gets activated when you enter.... The only confusion that I can think of would be with visas like that involve "leave to REMAIN" which you can apply for after you've already been living here - those don't need to be applied for outside of the country and you don't need to activate them at immigration. For example, I was here on a student visa (which you get while you're in the US) which was activated when I moved. Then I got married and applied for Further Leave to Remain, which I applied for from the UK and didn't need to leave the country to have validated.
So you need to get your unmarried partner visa in the US, come to the UK, and then deal with things like Indefinite Leave to Remain once you've been here for the required length of time.
Does that help?
