Living in London surrounded by English, your child will no doubt start to speak it. English is everywhere; on the streets, the radio, the tv, at nursery, at school so we can almost definately say that they will learn it. Every child is different and the time it takes may be sooner or longer than the next but they will be able to be called a Native Brit.
Spanish is one of the most spoken languages alongside English (which your toddler will have) and Mandarin. South America is becoming one of the most important emergent economies. Speaking Spanish will set your child at a competitive advantage.
Toddlers brains are developing rapidly in the toddler years and they have been proven to cope well and even outperform those that do not study a second language.
Imagine your child will not have to study Spanish but learn it through fun games and music while they are not in aware that they are learning. They will thank you one day!