Start the game with ultima ABCD[Alt]255 (hold down [Alt] and press 255) and you will get a cheat menu.
Easy Experience
When you first speak with Batlin in Britain and ask to join the Fellowship, he will ask you to run an errand for him. In order for this cheat to work, you MUST say "No" to him when first asked.
After refusing, speak with him again and accept the errand. He will give you a sealed box to take to Minoc. As long as you do NOT deliver the box, this cheat will work.
After getting the box, talk to Batlin again. At the end of the conversation, he will ask you to deliver the box again. When you say yes, everyone in the party will gain 100 Experience Points. You can repeat this as often as you like, but only so long as you don't deliver the box.
Free Expendables
This cheat is used in conjunction with the "Free Experience" cheat involving Batlin.
Once you have enabled that cheat, open a character's inventory and pick any item that has a quantity -- gold, arrows, reagents, etc. The item has to pull up the 'quantity' check-box when you move it. Put this item on the ground, and select the entire quantity.
Once you've done that, talk to Batlin. When you accept the mission, not only will everyone get another 100 Experience, but the item that you dropped on the ground will be duplicated in your inventory with the same quantity.
(Example: Put 100 gold pieces in the Avatar's hand, then put them on the floor. Talk to Batlin, and the Avatar will have another 100 gold pieces in his hand.)
This cheat will work with ANY item that has a quantity (i.e., it shows a number when you click on it). I'd recommend putting a few barrels on the flying carpet so you can haul around all the gold, arrows, and what-not you stock up.
Free Flying Carpet
As soon as you get into Britain, head northwest until you reach the mountains. Follow the edge of the mountains around the north until you get to a dungeon on the edge of a lake. In front of the entrance is a large blue rug with eight chairs attached to it.
This is a flying carpet. If you double-click on one of the chairs, everyone will get on it and it will lift up. You can go nearly anywhere with this -- across the ocean, over mountains, wherever. The only problem is that it won't land in a spot that doesn't have enough clear space.
Once you have the carpet, find/make/steal some barrels and stick them on the back of the carpet. I think four or five will fit there. This will give you room to haul all the extra stuff you'll need and not encumber your party. (This rug must be VERY strong to carry eight grown men, a barrel full of gold, another full of beef jerky, etc.!)