Ofcom are going to put in a maximum roaming charge when abroad I read the other day due to the amount of people not realising roaming charges or where they didn't turn data off or forgot.
They should simply turn it off when the limit is reached, as for it taking 24 hours to update that isn't your fault as a customer, that's their old fashioned systems. I see my data allowance updated within 5-10 minutes on my network and there's no reason why O2 or Tesco's systems are so antiquated. I would say they are to blame for you going over as their systems are always too far behind.
Personally I would sue them in the small claims court, as they can't charge you for a failure in their systems being a day behind and you using the allowance that was still showing on your account.
I would pay under protest and have them mark that you paid under protest and then take them to small claims court. It's unreasonable for them to charge you when they didn't notify you that you were over allowance, either by text or on your account.