First of all, are you sure it's for a cervical screening? It's women between the ages of 25-64 who are eligible for free tests. However, if it is for a cervical screening, there is absolutely nothing to worry about. It may be slightly uncomfortable but not painful. Thirdly, millions of women have them done every 3-5 years and it can detect changes which could be dangerous in the future, so it could save your life.
And finally:
"If a woman has never been sexually active, then the research evidence shows that her chance of developing cervical cancer is very low indeed. We do not say no risk, only very low risk. In these circumstances, a woman might choose to decline the invitation for cervical screening on this occasion. If a woman is not currently sexually active but has been in the past, then we would recommend that she continues screening."
So there's no need to panic.