OK, so after reading comments, i've decided to make this thread.
If you place an order using a voucher, they take the full amount, then the refund the value of the voucher (e.g £30 off £60, or the more recent, £25 off any spend). So no need to worry just yet.
Hope that clears up any confusion/worries people may have. You may want to bump this from time to time so more people see this.