Well, if you are Jewish and you have your mind set on eating turkey or a ‘festive pudding’ at this time of the year, you’d better eat a kosher one! And if there is a demand for it (as there apparently is), why shouldn’t shops promote them?
Most people do not regard eating ‘festive puddings’ or a turkey at Christmas as an expression of being Christian but as a cultural affirmation of being English; it is what people eat over here at this time of the year.
Influences from other cultures often help to strengthen our own identity, rather than weaken it. For example the giving of presents (instead of Chanukah gelt) during Chanukah, clearly been influenced by present giving at Christmas. Synagogue services are often better attended on ‘Shabbat Christmas’ than other Shabbatot, not because our members believe the day to be any special, but somehow they feel a need to affirm their Jewishness on such an obviously Christian day. Thus, eating a kosher turkey or a ‘festive pudding’ may be to some the ultimate expression of being Jewish in a non-Jewish world.