Hi there it seems you've topped up with credit rather than a goodybag. When using credit data is charged at 5p/MB so just 200MB would have used the full £10 worth.
Please in the future make sure that you purchase a goodybag rather than credit, it is a £10 goodybag that lasts for one month and includes 6Gb data, unlimited calls and texts.
On this occasion please contact an agent and let them know you brought credit by mistake and not a goodybag, they should be able to swap it over for you. They will take upto 24 hours to respond.
If you still require assistance, please let me know by clicking Reply below my message.