ppron747 wrote:No you don't - the UK has no objections to dual nationality so far as British citizenship* is concerned.
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*I say British citizenship because there are two other categories - British Subject under section 30 of the British Nationality Act 1981,and British Protected Person - where the status is lost when the person acquires another citizenship. But there is no problem so far as British citizens, British Overseas Territories citizens, British Overseas citizens and British Nationals (Overseas) are concerned.
It's also worth observing that while British Overseas citizens do not lose British nationality upon acquiring another, they will lose any entitlement they may have had (if otherwise stateless) to registration as a British citizen under s4B of the 1981 Act.
And British overseas territories citizens (BOTC) may find their immigration status in their home territory is affected, even though they don't lose BOTC.
In general, if a British child has another nationality, and the 'other' country doesn't like one of its citizens holding another citizenship, there is *nothing* that can be done to remove British citizenship from the child until he or she turns 18. Whereupon he or she can renounce it voluntarily. Up to that point, the views of the parents or the 'other' country involved are irrelevant.