Yes. For an art and design related course an END is an appropriate course. Generally it is true that is is better to do A-levels because national diploma restricts your options at university (you can get into top universities with a BTEC END if you get distinctions across the board, but it doesn't prepare you well for the type of study,) but this in not the case for Art and Design courses, because they really care about your portfolio more than your grades anyway. Also the type of study in art and design is by it's nature more project based, as is the END and ND.

However, you might also want to consider doing A-levels including Art and Design and Graphics if available instead. This would leave you with at least one other A-level option, which you could take in a subject with broader applicability, like maths, English lit, or history. This would keep your options more open later should you want to move away from graphic design, and could be appreciated by employers. If you go the A-level route I particularly recommend a foundation diploma before degree as it will give you a better introduction to art education and art practise.

Another possibility is to do an A or AS level alongside the END to give you a little more breadth. Some colleges can accommodate this, but often this needs to be by distance learning. There are a lot of distance learning colleges in the UK offering A-level courses, for around £400. This takes some self discipline, but is also appreciated as evidence for that.

Coursera has some nice MOOCs in this area that you might like to look at. It is not valuable as a formal qualification, but it will improve your knowledge and understanding, and it is a good way to show depth of interest. https://www.coursera.org/courses?query=graphic+des... I would suggest you do the CAL arts 5 course specialization. It is free, but you have to pay if you want to get a certificate of completion. I wouldn't bother, because I don't think the certificate counts for much compared to the benefit you'll get in developing your portfolio and being able to talk intelligently about graphic design in a personal statement and at interview.

Never ever study for a BTEC if you want to go to university. BTECs seriously reduce your choice of university to those that offer courses that don't make any difference to your job prospects. Not all university degrees lead to a better job some don't lead to any jobs at all. To get the best choice of universities you need to study A levels.

BTECs lead to apprenticeships.