Indeed, 0870 in the UK is a national rate number, however it is treated more like a premium rate number, a bit like numbers starting with 09 in the Netherlands. Your DC card should have a geographical number on it (indeed starting with +44 1332), so Skype will only charge the regular UK landline rate. The Gold number is different from the regular number

Even for people in the UK is it in just about every case cheaper to call non-0870/0845 numbers. That's why there is a website devoted to this http://www.saynoto0870.com/

Here's another page explaining why they are so expensive http://www.moneysavingexpert.com/phones/0870-say-no