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I mostly prefer the indeterminate types of tomatoes (that keep producing all summer long) and there are so many to single out. I'm also not aware of what your choices in London might be.
Potatoes I'm familar with that grow well and taste great are Red Pontiac and Kennebec. Again, not sure if you have those available there.
Onions, anything in the "granex" line will offer you some mild/sweet onions. Also, any type of "bunching onion" will do you justice as well (they never bulb up but stay in the 'table onion' or 'green onion' stage...great for eating fresh or for cooking with!)
Maybe you can tell us more about your garden area or allotment and how much room you have. That would help to give you a more definitive answer.