Well congratulations on achieving such an incredible rate for your cruise! These kinds of deals are few and far between these days.
Costa have never, to my knowledge included the service charge in UK pricing. I believe you can now choose to pre pay it, but imagine most choose to leave it and pay later onboard. As others have noted, for bookings made in some other markets, gratuities are included in the fare, and so the fare is usually higher, to reflect this.
Service charge / gratuities on Costa are amongst the lowest in the cruise industry, and are mandatory. As a comparison , Royal caribbean charge US$16 per person per day, and Norwegian US$20 per person per day. I have generally found the service on Costa to be outstanding, and way above expectations, relative to the rates charged. As a result I often tip additionally above the mandatory gratuities.
I hope you have a fantastic cruise! I must admit to being rather jealous !