Between Nigerian/Ghanaian, Indian/Pakistani, Jamaican etc foods, London should be an easy place to find good food.
The authentic "English" food? I'd probably find some Black British and ask them where to find the best native English food. If a Jamaican breh is like "ain't none" I'm taking his word for it. Full English breakfast looks good, fish N chips etc., but I don't get the fascination with beans. Rather have Oatmeal/"parritch"/porridge and all the other English breakfast stuff...swap out the beans. Maybe beans to the English is like grits to Americans??