Yeah,
The only other way to be entered is as an external student at another centre. You will need to ring (or Google) and find out which centres in your area accept external candidates. Also make sure it's the GCSE with the same specification you're doing and not a different qualification.
First thing to do is to really put the pressure on them with your parents and you to let them enter you in, even if it's a late entry with a higher fee and maybe take a print of that pdf.
As a last resort, if all of those don't work, even though they should, do an IGCSE and enter an exam as an external candidate.