I'm British, living in Spain, and we're bringing up ds with English and Spanish at home, as well as Galician at school and sometimes out and about.
My sistuation is different to you as Dh does speak quite good English, although never with ds. DH's English has improved since ds was born, as he hears so much of it around the house.
Also in our situation English is a desired language in Spain - everyone wants their child to speak it, so we only have very rare negative reactions to ds speaking English as well as Spanish.
In your case, I'd go for it of course, but be prepared for some ignorant people to question you and say things like "who needs German anyway" etc. Just stick to your guns. It would help if you could find other families who speak German too. I had to be proactive, and set up an English playgroup in my city, which has been a lifesaver.
Ds has developed both languages at a pretty average rate. In our bilingual group there are examples of kids at all stages of the spectrum - as in everything children develop at their own rates.
It's actually the norm all over the world for people to be multi lingual - the UK is unusual in this regard.
Have a read through the threads on the language/bilingualiam topic to be reassured more. 