It gives us the sight of Arthur Daley on a tube! I always thought that the tube and tube stations seem such an integral part of London it's a bit surprising they didn't feature in the Sweeney, Pros or Minder more often. But here is Arthur on one, and then coming across as most un London like when he blocks the barrier for several minutes because he has lost his ticket.
Also, it is inferred that Dave forces Arthur into paying cash for his VAT. No more slate. Must be a new found assertiveness from Dave after seeing Arthur paying cash for the round in '9.9 Last orders at the Winchester' a few episodes back.
Also linking this episode to 'Last orders at the Winchester', it is a bit surprising that Field had no idea that Gloria's fella was one Ray Daley. Indeed he seemed to hardly know or recognise Gloria even though she was at Ray's side during the auction at the end of Last orders at the Winchester. These continuity issues suggest that 'Last orders....' was filmed/written after 'The Great Trilby'.
Good spot, doesn't mention filming dates for these 2 episodes on the ep guide, must be as you say, as Gloria's boat would be hard to forget!
As for the lack of the Tube in the series, maybe permits and fees were involved and Euston didn't want the expense. I lived in London at the time and knew lots of regular young guys who never used the Tube. Owning a car in London wasn't the palaver it is now. Insurance, road tax, parking etc were all very cheap and easily affordable and the city was very easy to get around, even the West End.