Its easy. Declare anything that isnt part of the standard specification. An optional extra is an extra, regardless of if it was fitted at the factory, by a dealer or by you.
I have always found its best to be honest with insurers, they will soon tell you what impact various optional extras/mods with have on a policy. If the premium start increasing significantly due to mods/extras, simply find another insurance company.
FWIW there was no increase in premium when I declared my small list of extras/mods on my Peter best 'classic' policy.
Do you know if your quote is for a 'classic' policy or normal?