The type of export depends of the type of meter you have installed at home. If you have a smart meter installed then it has a Metered Export type which means the actual energy that is generated, is measured and payment is made.
All other types of meters have a Deemed Export. This type of export assumes that the majority of the electricity you generate is being used at home.
This calculation has been decided by the Commission for Regulation of Utilities (CRU) with government oversight. There is no mechanism to change this amount except for the installation of a smart meter to record actual consumption and therefore the surplus generated.
Your energy provider has no role in determining this export value.