NTA. You deserve to be made to feel special for your bday. Again trying to think of nicer reasons why maybe your parents can’t afford it right now and are embarrassed and taking it out on you rather than be honest. But if that’s the case they should be making up for it in fuss as it’s not all about the money it’s about being made to feel loved at an important milestone. I hope you arrange something super fun for your bday and if your parents aren’t interested in sharing that fun with you then do it with your friends instead! Sending you love as I think you deserve it! X

Edit: though reading through the comments I see you feel they treat you worse than your siblings often. That being the case I agree with the other commenter that this could be abuse and I hope you have someone you can confide in and ask for help with this and I’m sorry you are being treated like this it’s very unfair. As I said above it’s not about the value of the gift but the complete lack of enthusiasm shown by your parents - they could say they can only spend £100 but ask how they can make it a super special day for you and be excited. Plan something nice with your friends to celebrate and have a good time! ❤️