Just for clarity for others on this subject from my experience. I put in an initial enquiry through Howden's website for a quote for a new (to me) van purchase.
The enquiry was picked up by Howdens Bristol who provided me an initial quote. However, after just adding an awning(!) as a mod and an additional driver, the premium for their quote shot up. After me asking, they then transferred me to the Thatcham branch, who including this 10% discount beat the Bristol branch quote by 25% (including the mods and additional driver).
Therefore, if you are looking for an insurance quote for a modified van (even if the mods are not performance related) through Howdens, it is definitely worth specifically speaking to their Thatcham branch, who clearly have access to different pricing from their insurers/underwriters.