Kid car seats, uk?

The laws prescribe that children under a certain age and height must travel in a fitted child car seat. what would the position be if anybody without child car seats gives you and your child(ren) a lift? or if you travel by taxi/minicab? technically could you be prosecuted for breaking the law? would this be likely?
Answer
Yes, the driver is responsible. a mother at my daughters school was prosecuted for not having a kid seat (giving another kid a lift from school so not seats to go round). i asked the question to a taxi driver when i needed to use a taxi in an emergency and was told that they were exempt.