There is no waiting period for adjusting status from fiance to spouse. So, if you enter the UK on the 14th, and get married on the 15th, you can send in your application for your spousal visa/LLR/FLR immediately after your wedding (once you have gathered all of the documentation, of course!). If you apply for your spousal visa in person (by appointment at Croydon or...Birmingham and Edinburgh, right?...I think those are the offices....), you should get your approval that day. If you apply by post, it seems that the waiting time is up to a month. So, theoretically, assuming you get married as soon as you enter the UK and apply for the spousal visa by post, you could be unemployed for up to a month.
It is easier to get married in the US and enter the UK on a spousal visa as you enter with immediate permission to work. If that's important to you, then I'd suggest you guys do a "quickie" wedding in the US in March, you enter the UK in July as a spouse with full working privileges, and you guys can plan the more elaborate wedding for later on.
Now, as far as how long it takes to find a decent job? Well, that's an entirely different story! Really depends on where you'll be living, what your skills are, and what kind of job you're willing to take. I think most of us have started out on "starter" jobs to get a feel for the place and have moved on to more specialised positions as time passed.
(There's also a debate as to the money-saving aspect of getting married in the US for the spousal visa v. marrying in the UK with a fiance visa. If you marry in the US, you'll only pay for the spousal visa. If you marry in the UK, you'll pay for your fiance visa and your adjustment of status to spouse. But you won't pay the airfare for your fiancee' to head to the US for your wedding. Just a matter of figuring out what works best for the two of you.)
Good luck!
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