Time line for me: (well what seems practical)
-not engaged yet but are under understanding will soon be
-she moves in end of November under fiancé visa (6 months to wed)
-they spend the pre-season together, she goes home to see family for Christmas while he is at Sandringham
-meet up for a vacation between Christmas and New years, one year anniversary of first trip they took. Possibly proposal then.
-Scotland with his family for New Years to tell family. Announce early January
Wedding end of May. May 26 seems likely date:
-Saturday so no concerns for not being bank holiday
-weeks after Kate is due so no worry about baby or her not attending
-before all the big events in June
Also allows for a good honeymoon before June. They could go for a 10-12 day honeymoon and be back for Trooping. She could make her first official debut, after wedding, at Trooping of the color. She would be able to attend Trooping, Order of the Garter, and Ascot as his wife in June.
In October they make their first big trip abroad married, to Invictus in Australia.
I am leaning towards St. George's. 800 people is more then enough room for such a wedding. Likely have seats left open. Unlike the Cambridges there wont be hundreds of politicians and other figures that aren't personal guests. There may be some required guests but certainly not that many.