As long as you are both planning on going to the UK to live after your wedding, your fiance will not need to have any type of visa to enter the US and marry you. Once you're married, you can go to the British Consulate office in the US, get a spousal visa and then come to the UK and work immediately.
The only way to work as soon as you enter the country (if your visa is relationship based) is to get your spousal visa from the US.
The only other option is to get a fiance visa from the US, come to the UK, get married in the UK and then apply for a spousal visa. With this process though, you can't work until you get the spousal visa either.