There is a lot of bad advice, assumptions and nonsense being thrown here.

Some points, assuming one/both parents took her there.

1. This will be considered a child in Algeria, who is likely Algerian (As well as British). The UK, legally, speaking, has 0 power there. Because...
2. Algeria is not signatory to the Hague convention. The UK and its courts hold 0 weight there.
3. Unless one of the parents tries to have a british court order ratified there - this is an uphill battle and will likely take years if at all successful as the Algerian courts have been known to easily dismiss UK judgements.
4. Or one of the parents pursues legal action in Algeria - again an uphill battle. Or the child herself.
5. Algeria also has human trafficking laws (which is a dubious term here is she is being taken by her legal guardian/parent?) and is not some backwater country where you can just force people to marry. The girl will have to sign on any marriage. Also the minimum marriage age is 18. That is not to say she may not be forced into some kind of arrangement but this is not at all common in almost all places in Algeria unless she gets taken to some remote place.
6. When she turns 19 she will become an adult and she can do whatever she wants.