Best growout size tank, and best aquarium stores in london/paris?

Hey guys, ill be taking a olympic games trip to london (yay wembley stadium!) and then will be in paris for a bit. My question is, what are some of your guys favorite aquarium stores in these place? Including if they have a ADA shop, or GLA ect,

And question numero dos i want to start a plant growout tank/shrimp, what would be a good tank size that you guys use?
Thanks!