Most British-born children of British parentage speak only English. Besides being unable to fully communicate with non-English speaking folk from the offset, they run the risk of becoming culturally Anglo-centric. Though a shared sense of national identity can bring people together, it can also further divide people of different independent nations from one another. What one is speaking of here is raising awareness that the world can be full of limitless opportunities for all individuals if we are able to communicate effectively with one another.
Learning Spanish in particular as well as English from a young age would diversify a child. One can suppose that there is less emphasis in speaking an additional language in Britain because much of the world can speak English. One reason for not learning to speak another widely-spoken language such as Spanish could be that if for example all English and Spanish speaking nations can understand one another’s official language then the relevance of speaking either language would merely be a matter of choosing to speak one over the other depending on which boundary one is situated.
But learning a second language can inform a child more clearly just how they inhabit a language. For example, there are different ways of speaking of oneself and others. In English when one desires to eat food, one may say ”I am hungry” whereas in Spanish one would say ”tengo hambre” literally translated in English as ”I have hunger” (which you might agree is not necessarily a nonsensical translation). If one said in Spanish ”I am hungry” i.e. ”Soy hambre”, it is tantamount to saying ”I am hunger (itself)”, and it would not make sense or it would be something of an exaggeration to say the least, unless of course that is what is intended. And also in Spanish there is the specific concept of a certain state (estado) of being which in the English language is distinguished to a lesser extent. In English one can signify ones geographical position by saying ”I am here”. The absolute literal translation in Spanish would be that one is claiming to be the location itself, so the correct phrase in Spanish is ”estoy aqui” (I am [situated] here). Because those who created the Spanish language took linguistic care to distinguish modes of being, as irrelevant as this might be to non-Spanish language speakers, it can still make them more mindful of the different ways we consider ourselves and our relation to the world.
The chances are, if a child has a basic grasp of English before they begin to learn Spanish, they would technically have a relatively large Spanish vocabulary already. To revisit a Spanish word mentioned above i.e. estado, this somewhat refers to ”state”. This makes one more mindful of etymology which is somewhat taken for granted in intuitive learning of languages. But there are many other words whose meaning are easily recognisable and therein memorable because of their similarity i.e. animal, similar, posible, albeit with different pronunciation and emphasis on a different syllable. This is because the English language though unlike Spanish is a Germanic language, has incorporated thousands of Latinate words. And once a child learns Spanish, they then can easily learn Italian and Portuguese if they so wish because each belong to the same family of languages i.e. Romance languages.
Spanish is the second most spoken language in the world behind ‘Chinese’. But the Spanish language is the official language of more territories beyond the Spanish mainland e.g. in South America (with the exception of Brazil which is of course a Portuguese speaking country), most of the Caribbean Islands, Andorra, the Canary Islands (Tenerife etc.), the Balearic Islands (Ibiza etc.) and Puerto Rico (the island north of New York City) to name a few. Being multilingual better connects people that would otherwise be estranged from one another, makes us better travellers and creates for oneself more employment opportunities.