Travel from New Magincia back to the Deep Forest. There is a shortcut: a mountain pass north of the apple orchard in Britain connects directly to Yew.
Use the Emp whistle and listen carefully to everithing the Xorinite wisps say. Then, use (double-click) the Orb of the Moons to the north-west of the Avatar: a red moongate will open and the Companions will be transported to the Shrine of Spirituality.
Shrine of Spirituality
The Time Lord explains you that the Guardian created a spherical generator that prevents Moon Gates and Moon Orbs from working correctly (he also reveals that it was him who summoned you to Britannia, but the red Moon Gate appeared in Trinsic by mistake).
According to the Time Lord, this generator is located in Dungeon Despise, under the mountains north-west of Britain. If you played any Ultima from 4 to 6, you should remember that Dungeon Despise is straight north of Britain, instead.
When you take the Moon Gate at the Shrine of Spirituality, you will emerge on Verity Isle (Moonglow). Your main transport (ship or flying carpet) is back on the mainland. The "Recall" spell is your best option to leave this island; otherwise, you have to take your chance with the Moon Gate. When it works, it should send you to Britain.
If the Avatar is at high level, with high attributes and good magical armour, the Moon Gates are actually quite reliable. Each one teleports to a specific one, in this order: Moonglow to Britain to Jhelom to Yew to Minoc to Trinsic to New Magincia to the Shrine of Spirituality to Moonglow again.
Return the notebook
Before visiting the dungeon(s) in the Serpent Spine Mountains, you could return the notebook to Alagner. While you are in his laboratory, remember to use his magic glass sphere, that shows important events from one day earlier.