Not all insurers allow mods: As a general rule, it’s the cheaper ones that don’t or get extremely fussy/expensive if you do try and add a mod to your policy. Companies like Sky, Adrian Flux, REIS etc are good as gold but you will be paying more for the privilege.
You’ll have to ask each one individually really, unless they’re a specialist. You should be able to do a general Meerkat-style quote where you can add on the mods you intend to do, and get an idea of how much the premiums would go up by.