The lowest sugar jam I have found is Clearspring in Ocado, or even from amazon. Look for the proper jam, it is a proper jam made with kerrageen, they do cherry, strawberry Rasberry, apricot, marmalade and blueberry.
The carbs For the strawberry jam are 36.7 g per hundred grams, compared to 56g for St Dalfour. It is Expensive but there is an alternative jam called meridian that You can buy from Holland and Barrett that is almost as low in carbs and sugar.