G-EMMA, its discourteous to do this, but with A/G its up to the pilots to sort things out. If you were on base and Mr Long final nipped in front of you then that is poor airmanship and not following the rules.
If it was good VFR and you could both see each other it probably wasn't unsafe
However I'm sure it was good training in being able to deal with a situation that is not normal and that is valuable. You will come across things like this and being able to deal with them is part of being a good pilot.
If it was me I would have said something on the radio and later when the other pilot was on the ground.
I once did the same when landing at an airfield that was A/G. I was on long final and the A/G told me the runway in use and did not mention any traffic. They also said OK when I said straight in on a long final. It was only when I got to 500ft and 1 mile final that I heard a twin announce he was turning final. I hadn't seen or heard him. I couldn't see him but knew he was behind me somewhere so I landed and vacated quickly. He went around and then left for somewhere else. It was a genuine mistake on my part and I would have apologised had I had the chance. Maybe I shouldn't have just come in on a long final, but to join the pattern would have meant flying a circle around the airfield as I was approaching from a 20 mile final, it was early morning and I thought I was alone.