In this case, I find the lines to be too strongly visible so I suspect it is not genuine ivory. But it is very hard to say from only a photo. Maybe Thinreadline can provide more close-up photos?!
Also please note that all British silversmiths used to mark their products with a mark indicating the year of manufacture.
Check it out to see what year was yours made.
http://www.925-1000.com/british_marks.html
http://www.925-1000.com/dlc_sheffield.html