All children should attend school as much as possible. There are very high attendance rates expected of all children within the UK and unexplained or long periods of absences can be a cause for concern. If your child is ill then make sure that you phone the school at the earliest opportunity and that you try and avoid keeping them off school for any longer than is needed.
Taking holidays during school time is frowned upon and you can actually get a fine for doing so. Not only may you be fined but your child could be missing out on some vital lessons in school that means they will have a lot of catching up to do when they are back or that they do not have all the information needed for exams etc.
If your child is going to be out of school for a longer period due to personal circumstances, then you may wish to consider home schooling during this time.