It depends how comortable you are with maths and simply, how quickly you can grasp new ideas and methods. Like one of the chief examiners of one of the examining boards said (sorry to be so vague), the people who are naturally good with maths (whoever they are) generally find Maths A level an easy grade. Whereas those who aren't naturally good with maths, often work hard but still struggle. Thats a general comment but you get the idea.
I wouldnt worry too much if you havent done higher tier, but know that you will have to work much harder. If you get anything below a B, you WILL find A level maths seriously difficult. Do you feel you're good at grasping new ideas quickly? Do you actually enjoy maths and problem solving? If you dont thats a pretty big obstacle to overcome because Maths is just practice, practice, practice and you wont do practice if you dont enjoy the subject. Dont pick any subject because it looks good, its important to get a good grade so you have to genuinely like it. The higher tier stuff will be covered again so it won't hinder you much.
Grab C1 and work through it. You'll struggle a bit not having a teacher, but in general if you find it very hard, you might want to reconsider. BTW C1 is easier than P1 so go for C1 because thats the current syllabus and apparently more "accessible."
I wouldnt call myself naturally talented with maths at all but got an A at gcse doing maths at the end of Year 10 and an A in stats in Yr11. I did AS maths and found it really difficult. Far more difficult than Biology, Chemistry or Computing which I also did last year. I remember I got an E or D in my P1 mock around January time last year. Not good. But I worked on it, got some extra help and now I'm on my way to getting an A grade overall.
If you're prepared to work at it then you should be fine.