We found this one on Sunday in Tesco (Silverburn, Glasgow). We were looking for a small suitcase to fit as a cabin luggage but they were all £10+. Then we found this Tesco Value set. The medium suitcase just fits into the airline restrictions for the cabin luggage size.
I can't remember the exact price but it was definitelly below £10 (I think £9,97).
So basically you get a full set for less them if you buy any suitcase separately.