Crumbs & Doilies have a Wandsworth bakery and they make delicious cakes. They made my wedding cupcakes, but they make all sorts. https://www.crumbsanddoilies.co.uk/
Konditor and Cook made my son's first birthday cake and it was gorgeous. I picked up from their Waterloo bakery but they deliver all over London. https://konditorandcook.com/
Otherwise, you can find some great cakes at Marks & Spencers that you need to preorder: https://www.marksandspencer.com/l/food- ... es-n-byr1n
Asda also has some great options in store.