If it were me, I'd do a guided walk, pick one main attraction to see and fill in the rest of the tim

My husband and I are going on our honeymoon air travel deals canada this weekend. We will be staying in Paris for 10 nights. My husband really wants to go for a day trip to London. I know that it would have been better to buy the Eurostar air travel deals canada tickets earlier than now but the tickets still don't seem too badly priced yet. Obviously, we are already booked up in the hotel, so we would have to go to London in the morning and back to Paris the same night.
Assuming we decide to go (trying to decide relatively air travel deals canada soon), we are planning on doing a hop on hop off tour. I've looked into tours of Westminster Abbey, air travel deals canada Tower of London and Buckingham Palace, which seem great, but it seems like a bunch of tours would add up on time. If you were only there for 12 or so hours, would you think that the individual tours would be worth it or would it be best to just have a general overview of the city? We're also hoping to have a meal and hopefully some shopping at Harrod's as well.
With only 12 hours you can't get more than an overview. There's so much to do that it's difficult to recommend a short itinerary, but you don't want to spend too long queueing to get in anywhere or sitting in traffic on a bus.
There's no way you could do all that you have listed, but hopefully you know that. I often suggest doing a guided walk with www.walks.com They go for about 2 hrs, usually a bit longer and you don't prebook, just show up. Obviously as it's a walk you don't cover a huge area but if you find one on the website that is appealing to you, I'd go for it.
With the other things, Buckingham Palace won't take long as it's just a look at the outside. Assume you don't have tickets to go inside? The Tower of London would take a half day at least. I actually didn't like Westminster Abbey much, but lots do. How about going into St Paul's instead? The inside is spectacular and you can climb to the top of the dome for views over London.
Harrods seems to be popular but I've no idea why. The only bit I've ever liked is the fresh food section. It's pricey though, like the whole store. You could wander down Regent St or Oxford St instead (avoid the crowds on Sat afternoons).
If it were me, I'd do a guided walk, pick one main attraction to see and fill in the rest of the time by walking, exploring and eating and drinking. If you are near Selfridges at lunch time, Square Pie is good for a pie, mash and peas. Very English.