If your site needs more power delivering to it, it is likely that your District Network Operator
(DNO) will charge you for the infrastructural changes needed to deliver that extra power.
Exporting power is no different. If the existing infrastructure cannot handle all of the theoretical output of the solar array being exported onto its network then your DNO will not allow you to connect your solar array without first upgrading the equipment. The cost of these upgrades can run into hundreds of thousands of pounds.
The good news is that if your site uses a lot of electricity, it may be possible to install some clever hardware and software that links the site demand with the output of the solar array.