This looks like the double flowered form of Saponaria officinalis, generally known as Soapwort and sometimes as Bouncing Bet. Possibly native to the UK or more likely an ancient introduction. The leaves lather up with water to make a thin soap so it may well have been cultivated for that purpose. Still used for cleaning historic tapestries because of its mild detergent qualities.

 Not planted much in gardens these days being an 'untidy' plant that wanders through borders hither and thither. I have the single flowered Soapwort in my back garden and the flowers are daintier than the shaggy flore piano blooms.