"I guess can you buy proper packets of 'realistic' gravy substitute these days ?"

Yes, easy as pie. Bisto gravy powder, the old gravy browning stuff, has always been veg/meat free. You can use veg stock cubes or marmite or vegemite for the stock.

Or you can buy lazy gravy granules, many of the ones purporting to be 'for meat' are actually veg stock based = check the labels. But if in doubt go for onion gravy granules for brown 'beefy' gravy and vegetable gravy granules for a lighter 'chickeny' gravy.