I'm going to Holland and taking my mobile. While there I need to call someone else in Holland who has his UK phone with him. Do I have to put international codes in front of the normal number?
No because your phone is already known to the network as a UK phone. Likewise, if you ring a UK mobile abroad from a UK landline it's just the same, no prefix. Don't forget though that you'll pay to receive calls whilst roaming abroad
That reply is incorrect. If you are calling a UK mobile from abroad, you will need to add the UK International dialling code, regardless of where the other mobile happens to be.