Just now it does seem that many registry office's have limited availability.
But I guess if October is the earliest date then that is the one you should grab.
Once married you can apply FLR there's not any problem as long as the application is made before the fiancee visa expires. Seems plenty of time to me.
If you really feel so uncomfortable and want to try to bring the date forward, consider getting married at some private venue.
Or even rush off to Gretna Green
