In the United Kingdom the term Maid of Honour or Matron of Honour, is used to address your chief bridesmaid.  

The title matron of honour is to be assigned when your chief bridesmaid is married or a mature woman. Alternatively, the Chief Bridesmaids role will carry the title of Maid of Honour.

It is a long-standing tradition for you to say thank you to your Maid or Matron of Honour by buying her a gift which will be presented at the wedding reception.

Traditional gifts will be personalised for free, engraving the gift with a special message to commemorate the day will make the Thank You gift something that will be treasured.

Chief bridesmaid or otherwise known as the matron of honor usually receives a more lavish gift than the other bridesmaids.
 
The matron of honour is normally responsible for organising the hen night and bridal shower. Consider spending a more on her for the extra mile that she goes for you.

Your matron of honour should have been there for you right from the announcement of your wedding day, she should have been your rock throughout all the stress and preparations including going with you for your wedding dress fittings and attendance at any wedding rehersals.