Can I transfer my flexible cash ISA to Zopa's Smart ISA and keep my flexible allowance?

Unfortunately not. Your flexible allowance is linked to your specific ISA. Therefore, to maintain your annual ISA allowance, any withdrawals must be repaid into the same ISA.

This means, if you withdraw funds from your old ISA, you won't be able to pay them into your Zopa Smart ISA without affecting your annual ISA allowance.