Re: Just got new phone and sim from Virgin, can I cancel the sim?
Yes, I think you can.
The "cooling off" period is 14 days, but I don't know if that's only for the combined handset+SIM contracts. If you can split them, you can cancel the SIM and only have to pay for any calls made etc that are outside the allowance.
At worst, if you can't cancel separately within the cooling off period, you can give notice on the SIM and only pay one month.