The chain, which operates 72 c-stores and eight Caffe Nuovo coffee shops in hospitals across the UK, was ranked 21st on The Grocer's Top 50 poll of independent grocery retailers. In 2006 it reported turnover of £36.4m and a pre-tax profit of £2m, giving it a profit margin of 2%.
WH Smith said the acquisition would "significantly strengthen" the presence of its travel division in the hospital sector. It currently operates eight stores within seven hospitals, but is keen to open more.
"The acquisition of UNS creates further growth opportunities for our travel business," said WH Smith CEO Kate Swann. "The hospital channel is under-developed and the combination of the experienced UNS management team and the WH Smith offer will bring benefits for customers."
WHSmith's high-street business has struggled in recent years, but its travel division, which operates stores in airports, railway and service stations, last October reported full-year like-for-like sales growth of 2% and a 16% increase in pre-tax profit to £36m, on sales up 6% at £338m.
The division aimed to increase the number of stores it operated within hospitals by an average of 10% a year, a WHSmith spokeswoman added.
The retailer had not yet decided whether to rebrand the stores as WH Smith, she said. However, UNS' headquarters in Leeds would continue to operate separately from WH Smith.
UNS is the first retailer to exit the Top 50 since The Grocer revealed its 2008 rankings last month. However, it is the sixth leading independent retailer to be snapped up in as many months. C-store chains Smile Stores, D&M Edgar, Rusts, Chalfont Foodhalls and two supermarkets operated by Northern Irish retailer Curley's have all been acquired by rival independents, co-ops or the multiples.
A top competition lawyer warned in November that convenience chains could become a target after the Competition Commission provisionally found Tesco's and Sainsbury's move into c-stores had not harmed independents.