Seems fair to me. Coffee costs like 10p a cup and you sell for £3.
They have zero staff costs as all run by family.
Coffee shops are extremely lucrative its why I have around 30 of them within 5 minutes walking distance
Rent, rates, insurance, electricity, water, machinery, maintenance, cleaning, taxes, staff costs (kids are staff) etc.
Like I say, it is entirely possible. At first glance it just seems a high profit margin.