celebrity is known for very quick embarkation. If you never sailed with them you will be surprised. I have not been a retreat guest but we selected 11:30 as our arrival time when we did the check in on the ap and were easily in our room on the ship by noon. The earliest arrival time is 11 am I believe and you could probably try to get on board at that time, they don’t even check what your boarding pass says. simply put, you can pretty much arrive at your convenience.
As far as when to travel to Southampton, that’s up to what you feel comfortable with. We usually prefer staying by the port but if you value a day in London more that certainly has merit. It’s a fairly lengthy (and expensive) ride, about 90 minutes and mostly along highways but we didn’t encounter any traffic. We stayed at the Harbor hotel and it was nice accommodations, just walked around the harbor a little. Not a ton to do there. Other hotels many people stay at are more in the town. All very close to the cruise port, 5-10 minutes away at most.