I met someone online and planning on meeting up (need help!!!)?

Let's make it short and sweet... I met a girl online and we have skyped and texted eachover... We live in the same country but I live in London and she lives in lowesoft... Now I have been there before... But when I was younger... And we decided That I was gonna go up there... But here's the problem... I dunno how I would say this to my mum that I'm gonna meet up with someone I met online who lives quite far away... I'm almost 18... So wouldn't it be my decision... I was planning on getting the money which I can do... But what should I say to my mum... I mean she sill go all defence and that... But we really do wanna see eachover... We have skyped and texted alot... So anyone no rude stuff please but can someone tell me something I could say to my mum where she knows I'm going to meet someone but with a twist in the excuse?... Like changed the story abit... Like a old freind who I recently found online etc... Please help me on this guys... Would mean alot guys :') x