Wow, just wow......can I even.....
Okay, look I understand you are wanting to do the apprenticeship with a big brand like EY (which is good getting big brands on your CV) as it will defo boost your CV. However, not completing the apprenticeship with them is not only a waste of THEIR money and time but also yours. Why do you want to go to uni when EY do apprenticeship degree's? Its like a backwards step...
The very least you can do is finish the apprenticeship. If you do not then EY will certainly not take you on in the future (would you take someone on who couldn't finish a course that you paid for?). You also underestimate the power that a big company like EY has...they are very closely connected to other companies and will share stories/data with others, who will gladly blacklist you because their friend at EY said so.
Or the other company will look at your as a candidate and when they do research about you i.e they will contact other company's and might know people inside EY to get an idea of your character/would they recommend you etc. Let me be clear, it will not look good for you! Sure you might be able to blag it but I doubt it in this day and age, employers are tending to do their homework more and more on candidates for jobs.
Hope this answers your question.