A lot of paint suppliers wont recognise the old formula system now. The code for Admiralty Blue in the ICI book is 6856 Arden Green is 4881 this is within the long codes but unless they are old school they wont understand it. Hope this helps you
I have a strong feeling that if you look in the engine bay along one of the inner wing struts you will find a piece of metal with a number punched on it. Clean that up with T cut and you might be able to identfy the number. The real issue is if the modern 2 pack system can match that code.
There used to be a Paint Matching Shop in Bexleyheath Kent - I think Lion Rd - I have used them years ago - dont know if they are still alive - they could match cellalous either on the car or by a big paint chip that could be sent by post. I will have a look to see if they are still there and update later.