--I understand your discomfort. When traveling alone, the time I had moments of regret is when I did not feel well. I had thoughts of: I should have stayed home with my husband, etc. But I only had those thoughts when sick, alone, and had no one to take care of me if need be. With your timing, you were sick the entire time; colds last 14 days no matter what you do. Because of this sickness worry, I always stay in hotels. Front desks can be a big help, IMO. When I arrive at my first hotel, I will make sure I have a ginger drink (whatever is like gingerale where I am, and bland crackers or biscuits so if sick I do not have to leave my room until somewhat ready). On my last trip I started taking Emerg-C two weeks before my trip so I wouldn't get a cold, because I was on a the RS tour and might not be able to stay in sick if I wanted. I brought it with me to take if needed. I passed it out to a woman who it fixed up in no time. First time using this product. I like it. I believe it helped, and believing is all that matters sometimes. And know where you can get a good massage or what ever helps you to relax...you probably won't use it, but good to know. Also, I list in my journal on various days what I can do that day, not what I have to do.
--Day flight: I'll never say never, but I am lucky enough to live in Boston, and I never take any flight, anymore, that isn't the day flight. If I didn't live near this flight, I would train or fly to an airport which had a day flight, stay at a hotel and fly to London the next morning. They are that good for those of us who can't get a good sleep on an overnight flight. I only watch the price of that flight. While my husband and I 98% of the time took the day flight, solo I made a different choice, once. The last night flight I took was to Sweden, and that is where I was sick for the first 24 hours. A day flight means you don't get the extreme fatigue, and I say pain, of not sleeping on an overnight flight and forcing yourself miserable thru the next day. When I am going beyond London, I take this same day flight and I splurge and stay at the Sofitel in Terminal 5, order room service and relax, before my flight to my destination the next day. I fly economy but, IMO, this system is better than business class, and including room service and hotel, costs much less than business class. I arrive happy, get a good night's sleep in a wonderful bed, and casually stroll over to my next flight.
--If I recall correctly, at least your first trip, I thought it was extremely busy and structured. I can't remember if your second one was so structured. As a single traveler I wander, I sit in parks, I watch people. I relax! I also see sights, but often only one large site per day, maybe 2, depending on my goals/mood, and I'll drop something if I'm not in the mood, but might add something I'm wandering by. Regardless, I'll return, I say. What I'm saying is I think you put to much pressure on yourself, rather than having the first goal to relax and absorb the atmosphere. London has some great parks and many with cafes. They are great people watching spots. Or have a cup of tea seated at the bar of a pub and visit with the bartender. If they are not busy, they usually start the conversation, I find.
--Don't travel during late spring through early September. It's not worth the aggravation with so many people at tourist sights.
--I landed at Heathrow the evening of the attacks you mention. So I understand to some degree what you were feeling...and I wasn't feeling ill on top of it. The atmosphere in London that night was eery and the news alarming. Had I not been flying through, I would have not listened to the news. I would have asked the front desk the next morning what area I should avoid, if any, and pretend it didn't happen.
--I think you will suddenly feel the urge to go again, just relax and wait for it. "Just relax" is my maxim.