If the interest rate is the same, it makes no difference to the cost, and it will be neater and a psychological win to completely pay one off, so do that. Then send a bit to your other card to reduce the debt on that too.
Is this your only debt? No overdrafts? The interest rate on those is even higher so stay out of over draft. You could pay for your petrol on the card that is emptied and then pay it off in full every month, as this will help with cashflow and boost your credit history.
For the remaining card, always pay as much as you can, and definitely more than the minimum.
If you've never missed a payment and are 'only' in around £7000 of debt, while working full time, you might be able to get another card, so it's worth applying.
You might also be able to get a lower rate personal loan to pay off some or all of the card, so try that. Maybe avoid Sainsburys, as I've found them to be less accommodating, but perhaps look at loans from your own bank?
Also have a look at the Moneysaving Expert budgeting section and get the weekly email for ongoing tips on getting extra money and saving by reducing costs especially credit card balance transfers and reducing bills.