I wouldn't rely on the 24h cancel policy with the Lloyds voucher. Unlike an Amex voucher the Lloyds one is completely lost if you cancel. Now it may be that doesn't apply to cancellations within 24h, but even if Lloyds agree to re-credit the voucher It may take an age for you to actually get it back in your account.

Personally I wouldn't risk it... unless others on here can attest to having received the voucher back instantly in this scenario?