Old Inactive Handicap | Golf Monthly

Not entirely sure, but they my have 'created' your Index from the 'best 8 of last 20' as that is now the way it's done. If so, they'll likely also likely usef the Course Rating (and Slope) of your previous course.
FWIW. Even casual rounds can become 'qualifiers' these days, though you need to register them as such beforehand.
Here's a link to the World Handicap System tht is now the way it works - based on the Slope System that the rest of the world has used for some time.
I'd suggest you have a chat with the Handicap Sec regarding the unsuitability of your current 'cap. You could simply agree one with your mates while returning real scores until it becomes realistic - after 20 'new' rounds registered.