You could offer deals on your gift cards? For example - someone can pay £20- for a £25- value GC? Or two £25- GC for £40? This can help encourage people to purchase if there is an offer by creating a sense of urgency to buy before the offer runs out.
In my experience, people generally tend to spend over and above the cost of the GC anyway, so although you may have sold them for slightly less than face value initially - you can claw some of the money back when they come to use the voucher.
Last year I gave a complimentary £10 beauty treatment voucher when they purchased a gift pack of products. It helped sell more gift packs and clear stock.
I thought the vouchers might bring in new customers but what they actually did was give the gift pack as a present and kept the voucher for themselves! I did put a use by date of March on the vouchers though so at least it made sure they were in again relatively quickly in the New Year.