Consolidation normally holds out the prospect of improved profitability. The opposite is true in the UK mortgage market. Nationwide's acquisition of Portman propels the combined building society into the number two slot in the UK mortgage market with a share of 11 per cent. Because both are mutuals, any excess profits are returned to members in the form of lower pricing, which means the new, larger entity could intensify downward pressure on pricing. They are happy to live with a return on equity a few percentage points lower than their bank competitors. 

FT Article