I have not done a top deck however I have travelled in Europe by bus. The bags are a perfect size. It is good to have a bag with a handle and also back straps to use as a backpack. It can be tricky getting around in London with your bag as most of the tube stations don’t have lifts and you need to use the stairs. If you are travelling with someone than it is easier as you can help each other Cary luggage up stairs etc. I don’t think topdeck mind the size of the bag its the weight they care about. My tips, pack light as you are the one carrying it plus you want room for all the things you will buy. I am going on a tour in August and plan to only 1/2 fill my bag. Last time I was away I made the mistake of taking to many clothes, you wont wear them. Take washing powder and less clothes.
Hope this helps!