I would recommend charity shopping! It takes a little longer but can be well worth it. Most of my formal clothes are originally M&S or Next, but have picked them up in charity shops for less than a fiver. Most formal clothes are machine washable nowadays anyway, so as long as you pick and choose secondhand clothes can look as good as new.
Remember that if you're going to do medicine, you'll need three or four different sets of formal clothes to wear for placements in university anyway. It's worth dropping into charity shops on your way past over the next few months and just keeping an eye on what they've got. :]