Is there any way to find out what actual values I'm getting from different armour and/or weapons? For example, "excellent breastplate" vs "worn chain of protection" ?? I know I can just guess all I want, but is there any way to actually know?

I second this inquiry!

Found this somewhere ...

Weapons come in the following calibers: (from worst to best)
Swords - dagger / short sword / long sword / broad sword
Maces - cudgel / light mace / mace
Axes - hand axe / axe / battle axe
Missiles - sling / bow / crossbow
Shields - buckler / small shield / wooden shield / tower shield
Armor - leather / mail / plate

Weapons/Armor/etc come in the following grades: (from worst to best)
badly worn / worn / servicable / excellent

Yep, that's pretty much easy to guess. What I want to know is how the magical addons fit into this; lesser/normal/greater protection/accuracy/damage, etc.

So, as I wondered originally, how does (to reference a recent decision I had to make) "excellent breastplate" compare with "worn chain of protection" ??

The "of protection" enchantment adds some damage resistance but it's not clear how much this adds (even the clue books do not explain it).

Enchantments aside an item of a lower tier is better than a higher one if its quality is sufficiently better (e.g. "serviceable chain" would be better than a "badly worn breastplate"). Higher-tier items add a bonus of +4 or whatever but the condition determines how likely it is to do its job; a sufficiently worn item won't work often enough to make its bonus worthwhile. The hint books probably explain this better than I have. :)

Item quality is only a problem early on; if you are exploring thoroughly you'll soon come across someone who can repair any item to "excellent" condition (unless it is "ruined" or "broken"), and you only need a few gold per item. Items only become noticeably less effective when they are "worn", and it takes a very long time for an "excellent" item to wear down that far, so you won't need to return for repairs very often.

You can also learn the repair skill yourself but you run the risk of breaking the item if your skill isn't high enough and the gold you'll be saving is of little use for anything else. I'd recommend using your training for other skills instead.

EDIT: Ultima Underworld II adds a spell that you can use to repair items, giving magic users another option for getting things repaired.

Yeah, it's actually something that you learn through intuition and experience. I have not seen a single guide out there that could offer concrete numbers.

In addition to Lusche's list of weapon conditions, here's the list of enchantment levels:
(of) Minor (Protection)
(of) - (Protection)
Additional
Major
Great
Very Great
Tremendous
Unsurpassed

I'm carrying a buckler and leather leggings of great protection instead of a tower shield or chain leggings for saving weight. Works well enough for me. I'm on dungeon level six in my current playthrough, and these enchanted pieces of light armour still do a good job so far.

I found a dagger of very great damage - I wonder if it's better than a jeweled sword... There's no way to tell but by trying. :)

What I would like to know: What exactly does the "toughness" enchantment do? Actually... No, not really. :) Note: "Item" means armour or weapon. Once, I was wearing a perfect crown of great protection for some time, until a feral troll decided to use my head as a drum. The crown went directly from perfect to tarnished. And jewelry can't be repaired. With the enchanted light armour pieces I collected so far I did decide to spend some training in repair, as that saves me running the long way back from the deeper levels to have my stuff repaired.
Also, it saves time, as you have to wait for the completion of the repair of each singular item.

... and even the "experience" is of limited value, since you don't get any measurable info on what damage each blow did... *any* weapon is capable of doing virtually no damage, or anything in between. So observation doesn't give all that concrete of guidance. I think Ketzerfreund found the key word: "intuition"... and to some extent, just picking one and being satisfied with what you got...

I think the only way to answer your question would be to actually open the game files and search for the various values, but I'll be darned if I know how.
I did a somewhat extensive search online, but apparently no-one bothered researching this, since after having visited a lot of different websites, I simply could not find anything.
Bottom line, unless someone actually does that, the only thing you can do is get a weapon you are skilled into and the best armor you can find, and try to keep them in top condition.

I believe it pertains to durability - its ability to take punishment. So, it is quite possible for a leather vest of unsurpassed toughness to last longer than a standard breastplate. I personally do not care much for those items with this type of enchantment. I almost always opt for the protection enchantments as they work great with lighter armor. Toughness enchantments would only work best for plate armor. I almost forgot - ratcheting up your Defense skill to near max levels makes you very hard to hit. I was able to get through a lot of the lower levels with only a few special pieces of equipment - sword, shield, dragonskin boots, maybe some forms of protection from magic. It is surprisingly effective.