Congrats on your engagement!
If you want to get legally married in the US in August, then the marriage visa is not for you (as Larrabee has explained). If you want to get legally married in the UK, you're looking at some strange timetables to do the marriage in the UK, get your FLR(m), and make it back in time for a wedding party in the states in August. Also a lot more headache.
For example: if you arrive on a fiancé visa on May 1st, you have to wait 7 days to give notice to marry. You'll then undergo a minimum of 28 day wait, but it could be as long as 70 days, to get married (which, if that happens, would put you into August). You'll then have to sort out the FLR(m) likely in person ASAP to make sure you have your passport and BRP to travel back to the states for the wedding party. There's a lot of places in the process where things could go wrong in the timing and prevent you from making it back to the States for your pre-planned celebration. Theoretically, I'd imagine it's achievable but I would strongly advise not taking the risk and stress on! (Also, experts here may have much more insight to this scenario than what I've learned thus far)
As suggested, it might be worth looking into coming over for a visit for a few months (say, May-July) to test everything out and then apply for the spouse visa from the states right after you marry. It will mean a little bit of time apart potentially, but it's probably the most straightforward route.