I'm pretty sure that you can just pop into a Virgin media store and pick one up from there. It would save you waiting for delivery and they could activate it for you but I would advise that you phone your local store & check with them first.
Alternatively give the Virgin mobile team a call on 789 from a Virgin mobile or 0345 6000 789 from any other phone & request a new SIM of the correct size.
You just have to pay for postage: 99p standard or £2.99 for next day.
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