Out of historical interest, I would like to play Ultima VII without Forge of Virtue. It would be nice to be able to optionally install the game without it.

However, as that is not possible, I’d like to know if the expansion adds anything else except the extra island? That is, can I get the original experience by simply ignoring the quest? I haven’t played Ultima VII before, so I dare not delve too deeply into the internet to find the answer in fear of spoilers.

The expansion was also how they applied bug-fix patches at the time, which is probably why it's installed by default.

To the best of my recollection the only differences are that the island gets added, a few new dialogue entries are added to Lord British, and an extra ship (usable only with a deed given to you by LB when you talk to him about the island) is added to the docks in the town of Vesper. There may be one or two other NPCs who comment on the island's appearance, but otherwise nothing should change content/story-wise and you ought to be able to play through without visiting it.

Yeah you don't wanna miss any patches it may have included. Also Serpent Isle begins with the assumption you've done Forge of Virtue.