The DG Regulations require you to ensure an appropriate metering installation is installed.  While strictly speaking, with your retailer’s agreement you could undertake to do this, e.g. by contracting directly with a metering services provider, the simplest approach is to let your retailer make these arrangements. 

These days, both technically and from a cost perspective, there is little if any difference between a modern simple import meter and a combined import/export meter.  Retailers arrange for metering services companies to install and maintain millions of metering systems nationwide.  As a mass-market retailer, they will likely be reluctant to agree to a one-off special case where they provide the import meter but you provide the export meter (bear in mind that there is currently no compulsion for any retailer to agree to purchase your excess generation from you). 

Retailers’ costs to track and operate special case arrangements added on top of your costs to provide a standards and rules compliant export meter would make this a complex and inefficient scenario from everybody’s perspective.  A single modern import/export meter managed centrally is both an administratively simple and cost-effective solution.