Although you trained in a particular subject for your PGCE, your QTS is not subject specific. You can both ask, and be asked, to teach other subjects.
Obviously, you will be able to do this to a higher level with a degree in the subject. I don't envisage any problem with a degree undertaken through part-time or distance learning compared with a full-time one. You won't receive any funding, so will have to pay full fees for any second degree.
There is no shortage of history teachers so talk to the head of history at your school about the possibility of some key stage 3 classes; but your department may be reluctant to let you make the switch.
If you have a question for John Howson, please email firstname.lastname@example.org