Don't invest unless you're prepared to lose all the money you invest. This is a high-risk investment and you are unlikely to be protected if something goes wrong. Take 2 mins to learn more.
With the Bank of England holding interest rates at a record low, why would you choose to open a cash ISA?
If you are a non UK resident, you may be able to reclaim a proportion or the entire 20% withholding tax attached to the interest paid.