I think Nationwide have realised that customers are not happy with 0845 numbers and that they have customers abroad, I have always used the 080030210 number if I couldn't find an alterntive.
I have found that this appears to be the best method -
Contact the main switch 0800 302010
Ask for the relevant department
Before you terminate the call ask for the direct dial 01 number because you cannot contact them via 0845 numbers or tell them that you need to access the service when you are abroad
Both methods have worked for me