I have agreed values on three classic cars:~ two Volvo- one Riley.
Bishop Calway-Peter S Taylor.
https://www.bishopcalway.co.uk/
Straight forward and all they required was a full description plus photographs of all the car-Exterior-Interior etc.
Most company's will have the car market values on file and I believe that Hagerty is used:~
https://www.hagerty.co.uk/valuation/
you can do the same for further guidance.
Recently made an adjustment to the Riley cover as the market had changed and it became under insured- again the above procedure used without any problems.
As previously advised I would avoid paying for an independent valuation until you know what the Insurance requirements are.
Good Luck
Bob.