Terry's British Leyland cars (and others in the show) are a nostalgic part of the viewing experience of this show and Terry & June. There's something quite fitting about them that feels right for the series - they're comforting, terribly British and rather bland. He started out with a K-reg Maxi (which would be '71-2) which has regressed into a K-reg Austin 1800.
I did spot a newer Maxi in the background in the boat episode and thought it might have been Terry's, but it seems not. Now he's driving an R-reg ('76-7) Princess, so we've pretty much caught up with the model he drove in early Terry & June.