A lot of people choose to get a Halifax share dealing account, it seems the most straight forward way to go there are more details in the longer thread linked above. Don't worry about whether or not you get your card in time. As long as DLP have the evidence that you've got the shares they'll send you out a temporary certificate which will be valid for three months. I was there last weekend with a temporary certificate and had no problems at all in getting my discounts.
The only thing was that because it wasn't a card that could be swiped at a till point they'd have to call for a manager to come and take the discount from your cost. They were very quick though, when rang they answered straight away appeared not long after apologising for making you wait and putting it through. Much better service then you get in other places when you wait a week for a manager to show up