As Mamya points out, 'blackberry' and 'bramble' are virtually synonymous terms when it comes to jam. (My mother always made bramble jelly from the multitude of blackberries that my father and I picked on local heathland).
Mackays Seedless Bramble Preserve is £1.65 for a 340g jar in Tesco, Morrison's and Sainsbury's. Morrison's also stock their own brand, at just 65p for 454g.