ISAs are good for taxpayers because they generate interest that is tax-free, and ISA rates used to be quite attractive because banks/building societies used attractive ISA rates as a headline grabbing device to draw in new customers to their bank. However ISA rates are now pretty low and usually include lots of catches such as attractive bonus rates which only last a year. Furthermore, for someone like you, Molly, who is likely to be a student for at least 4 more years, a tax free account isn't relevant as you are very unlikely to earn enough each year to be a tax payer anyway.
You need to find a cheap day to day account for withdrawing cash, using a debit card and paying bills such as mobile bills by direct debit. Look for one with no fees and perhaps some flexible/very cheap overdrafts in case of emergency. Perhaps keep £500 in there, £1000 at most.
You then need a savings account which pays a good rate of interest and where the money is accessible but not so easily accessible that you continually dip into it and fritter it away. You should keep as much as possible in here because you will need it one day.