I doubt you can 'insist' where the inspection is carried out. Inspection is often carried out by independent, self employed, engineers who may inspect several other bikes on behalf of various insurers on the same visit. They will want it to be a neutral and suitable place, not subject to potential undue influence from the bikes owner being present.
You may get them to agree before it goes that you have first rights to salvage, if its written off , which should improve the chances of getting the bike back,even if its not repaired. Its then down to arguing the payment of the claim rather than the possession of the bike.