It is because of the turbine. To connect the turbine to the grid, and be able to 'freely' consume the generated units, it all had to be done at HV. If the turbine was connected to it's own HV metered supply, you could only sell back to the grid, and not use the units yourself.
It is a bit political as in effect, one company owns the turbine and one the site which it provides the electricity to, the 'free' electricity being invoiced from one company to the other at the same unit price as the electricity from the grid, so the site in effect sees no benefit from the turbine, but the other company makes the profit from selling to the site, and the FITs payments.
All this would be completely fair if there were no losses through the substation. This is why I have now fitted a third meter to record the units at LV.
The turbine is of a size and of a distance from the site, where connecting at LV would be impractical.