Lie your ass off. Your parents might not know for certain you have a partner, but they might suspect and try to trick you into admitting it. If your parents say something like “we know you have a gf” deny everything and put on your best poker face. If they knew for certain you had a partner they would have specifics and details.
Don’t be that person who lets guilt, obligation and fear get to them and says “I’m an awful person for lying to my parents” and do something like come clean to before you’re independent because you feel bad for lying. No, lying is a survival tactic. 8/10 times telling the truth only results in ultimatums and threats and the end of your relationship. If the guilt is getting to you seek therapy if it’s an option.
TrustNo1 Mulder was onto something. Family friends are not trustable. In a moment of weakness (or malice) they may tell their parents about your relationship and in turn their parents will tell your parents. Family your age ie. cousins and even in extreme cases siblings are equally not trustable.
You and your GF should consider staying away from parks in your area. The U.K. parks in my area are swarming with Somali aunties and uncles because us Somalis have hereditary health problems.
I have always been hiding all of my relationships from parents, it's been 10 years of dating experience and my parents know none of them but it really depends on your situation, some tips can't work if they really control everything you do.