Surely if you get cashback (local Waitrose still does it, not sure about others,) it will just show as £126.57 to the supermarket, not £76.57 + £50.00? It's how it shows on my statement if I use my debit card, a single transaction, but I haven't tried getting cashback on a credit card, and I don't know if they'd do it on a cc.
On the supermarket receipt, it shows two lines - cashback, then change for the same sum - the end total would still show as £126.57, and that's what the cc company sees, not the detail that you bought spuds, loo roll, shampoo and cashback.
I wouldn't use a cc to get cash from an ATM, unless a dire emergency, as that's where you get charged for the withdrawal, and get higher interest rates than on purchases. But I think but do not know that if your supermarket a) does cashback at all and b) does cashback for credit cards (rather than debit cards), you would be okay. But I don't know for sure.