Can I add funds, and how if I can. If I had a 10 pound Google play card and I needed 14 pounds on it can i add 4 pounds to it?
Answer is to your question is No, you cannot add an amount to a gift card after purchase. You'll have to purchase another gift card & redeem both in order to accumulate the desired amount.
As per the Comments
There is no information which actually states this. but this document says that user can use multiple gift cards to complete the purchase. What I've seen is, third party seller sells the gift card & updates the gift card number with the amount to google. so I don't think there is any provision to update amount later. otherwise they would have stated that way. https://play.google.com/intl/ALL_ALL/about/card-terms/