Where to put cash whilst waiting for the new fiscal year?

So I’ve maxed out my S&S ISA (uk based) for this year and have saved enough to max it out again for next year. In the uk you are only entitled to put 20k per year due to no tax and the year starts and ends on the 5th April. My question is is there any way I can make some money with the cash (20k) whilst I’m waiting to deposit into the ISA? Figured I may as well not let it lose value.

For interest my ISA is 100% S&P 500.