Faint lines...

Abbie
So I first got a very faint line about 4 days ago at 9dpo. I am now around 13dpo because I ovulated late. I was due on yesterday. My lines still aren't very dark, they are darker than at 9dpo but they're not as dark as I thought they'd be considering I am officially late, anyone else had this? I'm worried my hcg levels aren't going up as they should. 
I live in the UK and I don't think my GP would do blood test without a lot of persuasion. 
Opinions please!! Xx