I was in the same boat a few years ago - all miles concentration was on bmi and I was shopping at Sainsbury - but since swapping to Tesco - in the last 3 years or so I've earned close to 200,000 BA miles mainly from Tesco - and I'm not buying things that I don't use either! Just stock up on wine when necessary, make use of the Shopping codes when they are available - and they soon add up. As others have said - do try to avoid spending your miles on Economy flights.

Also - you don't have to decide what to do with your Tesco vouchers just yet as they are valid for some time (1 year I think) - so you could later decide whether to convert them to airmiles or BA miles, or something else.