I live in Spain, am a translator and speak fluent Spanish, but I speak to my dd only in my native language, English.
To reiterate what others said, it's important that I speak to my dd in English all the time, because she has Spanish all around her, and will be immersed in it every day when she goes to nursery school.
I used to speak Spanish a bit to dd in the park in front of others, but now I think I'll stop, because I don't want to confuse dd, and if others get offended I don't give a stuff
That said, most people are very positive about it in the park, and like to join in and get their dd to show off the English they know. It's sweet. Sorry to be cynical, but I bet they wouldn't be so friendly if we were Moroccan and speaking Berber or Arabic.
I had people give me very pointed dirty looks in my home town for speaking Spanish near them to my ex. They assumed I was an immigrant and were disapproving of me speaking a different language in their precious presence, and probably of me being there in the town at all. It's sad, but I just see it as small-mindedness, ignorance and fear (are they saying something about me?)
I agree with Cory, many immigrants do not have the same advantages in terms of learning English before they get to the UK as say an ex-pat who has ready access to language classes here. I'm sure most learn the language quickly enough through LIVING in the UK.