No it's not against the law, plenty of people have them. I was going to get one myself. Personally I think they improve the look of the van and will save you a minor repair when someone reverses into you in Tesco's carpark
However I think you have to weigh that up against the fact that they make your van much more dangerous to pedestrians. It's a personal choice. I've never run into anyone but I know it could happen and if I caused someone a serious injury or even killed them because I had fitted a bullbar I couldn't live with myself. It's a tricky issue.
Steve