^^^ Tesco's plans, as announced so far, AuntLydia, are to open two Jack's stores in mothballed Tesco stores but to build purpose-built units elsewhere.
If people are offered the choice between Tesco and Jack's, they'll probably go to Jack's if they're only after the lowest prices on a limited range of goods but to Tesco if they want to be offered a far greater choice of food items, or if they want to use the restaurant, to buy clothes, to put fuel in their car, etc.
I've worked on transport surveys, monitoring the movements of cars going to and from supermarkets. A large number of people who shop in Lidl or Aldi then head straight to a 'full range' supermarket (such as Asda) to top up their shopping requirements with all of the items which they couldn't get from the discount retailer. So there might be plenty of people who start their shopping trip in Jack's but who then go on to Tesco.