This petition was rejected

The Government e-Petitions Team gave the following reason:

Child marriage is not legal in the UK, so we're not sure what action you'd like the UK Government or Parliament to take.

British law already applies to all citizens in the UK. In a response to a previous petition, the Government said:

There are a number of Sharia councils in England and Wales which help Muslim communities resolve civil and family disputes, by making recommendations, which they hope the parties will abide by. They are not part of the court system in this country and have no means of enforcing their decisions. If any of the decisions or recommendations they make are contrary to national law (including the Equality Act 2010) then national law prevails. This is no different to any other council or tribunal - whether or not based on Sharia principles.

You can read the full response here:

petition.parliament.uk/archived/petitions/48352

It is therefore not clear what action you would like the Government to take, since Sharia councils already operate under British law in the UK.