Hi, we'll be doing the same, I'm English and my husband's first language is Italian. We will be moving back to mainland Europe soon after the baby is born, either to an Italian, French or Spanish speaking country (I speak French and hubby all 3).
Plan is that English will be the language of the house, but if DH is speaking directly to the children he will speak in Italian.
I worked as an aupair in bilingual households and it's very important that you stick to the language that you're meant to speak with your child. One father would start off speaking English and then remember he was meant to speak French and switch halfway through the sentence (plus his English wasn't great anyway) and the poor child was so confused and had terrible speech delay. In another family, the mother only remembered to speak to the children in her native Swiss German when she was disciplining them, which unsuprisingly led to bad association with the language for the children and they refused to speak it.
I think being persistent but not pushing it is the key. DH's mother always spoke Spanish to him, but he would reply in Italian, however as an adult his Spanish is still fluent and now he enjoys speaking it. As a child he just wanted to be the same as his peers.
I would also suggest have bedtime story tapes in the language that isn't the main one the children will hear (ie Turkish for you if the children are brought up in the UK), this really helps them subconciously pick it up.
I shall be learning as I go along too, so it will be interesting to hear from any other bilingual families for their tips!
Good luck