From Wiki

• If you are moving back to the UK with your child who was born abroad but who is a British citizen by descent from you, it is strongly recommended that you bring your child into the UK with a British passport.

• If you travel to the UK on holiday, it's okay for your child to travel on a foreign passport.

• But, if you enter the UK with the intention of resuming residence in the country, you may run into big problems at your port of entry if your child is not entering the UK on a British passport.

• A forum member reported that she and her husband were given a difficult time at the airport when they brought their foreign-born child into the UK on a foreign passport. Immigration officials eventually relented and gave them temporary authorisation to bring their child into the UK. But, in the following weeks, there were all sorts of bureaucratic chores that the couple had to do in order to rectify the situation.