Hi! You asked about dinner reservations. Well, there are two school of thoughts, take your chances and you may be very well able to get a table. However, I try to make reservations whenever possible. It can't hurt and it could save you a headache of a long wait. I would make reservations especially if there is someplace you really want to go to. Yes, sometimes, it really didn't matter if I made reservations or not and I got a table quickly or the place was not that crowded but why take the chance.
Today, you can easily make reservations on the restaurant or pub website! Or you can ask your hotel to make it for you. It is so easy to do it online and it only takes a few minutes of your time. I found that many pubs are now taking reservations and found them very busy. I also like to make reservations in Paris. To me that is a must as many of the Paris restaurants are small.
I rather be safe than sorry. Many may not agree with me but I find it easy and quick and less of a headache to make a reservation. This way your not turned away and not have to hunt for a place to eat.
If a museum wants reservations like the Churchill War museum, make them. I was lucky, I happened upon the Churhill War rooms and there was a line but it moved quickly and I got in. But others have told me that the lines were very long and they were happy to have a reservation.
I plan on going to the British Museum and the National History Museum next spring and will go before they open to get on line. I also like to go to the most popular exhibits first as they get more croweded as the day goes on.
The best way to make things go as smoothly as possible is have reservations for museums, dinners, and anything else that requires a reservation. If there is a museum you really want to see, then get there before they open to be on line to get in as soon as possible. Do the things that are not as popular in the afternoons. Map out your days so you know where you want to go to first thing in the morning and then have a list of what else you want to see and what sites you want to see that are closest to where you will be after your first thing in the morning museum or site. This way you don't waste time traveling around but are closest to where you finish in the morning. I make sure where I am going in the morning and then see where the day takes me.
I also look at the pubs and resturants that are near my hotel to see where I would like to eat or stop for a drink on my way back to the hotel. I also look at the RS guide book to see what restaurants and pubs he recommends near my hotel or near a museum that I am going to. I will then look at trip advisor to see if people like those resturants and pubs. For my upcoming trip to London there are a number of puts and some are good to eat at and some are just good to have a drink per trip advisor.
I like to have an idea of the resturants and pubs to go to while in London ( and this goes for Paris too) so that I don't waste time trying to find a place to eat.
You want to look at google maps and map out where you going what is near by that you want to go to, where the tube stations are and where your hotel is and how to get from your hotel to whatever the first museum or site of the day is. I keep a folder in my IPhone and screenshot my tube maps from google to know what tube lines and stops I need to take to get to my first morning site or museum. It saves me time in the morning knowing what tube line and stop and how many stops there are inbetween.
No matter how much you plan things do happen, trains are delayed and lines are down but it helps to be prepared.
I also have an Uber app and used it twice in London when it was pouring rain and nitetime and it was easier to take an Uber than go to a tube stop with no umbrella.
Have a great time, hope this helps!