The deal is set to be completed in March and the stores, which all average 5,000 sq ft, will be refitted between June and August.
Locations for the outlets range from Belfast to Honiton in Devon. Sites have also been acquired in the Republic of Ireland, Scotland and England.
The stores are expected to stock 2,000 Marks and Spencer lines, including ready meals, fresh produce, cards and hosiery.
There are currently 143 Simply Food stores in the UK, Republic of Ireland and the Channel Islands. These include 40 franchise stores in railway stations, petrol forecourts, airports and motorway service stations. The purchase will take the portfolio to 171 shops.