Can I use instant cash ISA for shares in UK?

You are allowed to use the money in a cash ISA to buy a shares ISA. If you want to maintain the ISA status, you need to follow the procedure set out by the company that you're sending the money to. You have a limited amount (currently £20K) that you can put into an ISA per year, but transfers are not counted against that limit.

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The main advantage that there is a possibility that you might make some money.

The interest rates paid on cash ISAs are below the rate of inflation at the moment. So even paying no tax on the interest, your savings still lose value every year.

A shares ISA may make money, if what it's invested in goes up. But shares can go down as well as up. Nothing is guaranteed.