Huawei Ascend G300 (U6615) not using Three network
According to the MCC/MNC table for UK, your MNC code should be 20, not 15. Also this page suggests that you need to enter default,supl,mms (without spaces after commas) in the “APN Type” field.

If this still won't work, try omitting all MMS-related settings (MMSC, MMS Proxy, MMS Port) and entering default,supl in the “APN Type” field, then, if you get your 3G Internet connection working this way, add another APN just for MMS with all settings and mms in the “APN Type” field.