Drupal/Ubercart module tweak

We have an e-commerce site which contains both items with vat - and items without vat. At the checkout, customers are offered free delivery on orders over £120 including vat.

I need the system to calculate this cut off *excluding* VAT - which would be £100 excluding vat.

The site uses a flat rate shipping option, which is set up through ubercart conditional actions. Would it be possible to add some custom php into the conditional actions to drag out the ex vat price as a condition?

I have set up a staging site at http://cf-staging.dnsalias.org/ and can give you ftp and phpmyadmin access if necessary.