Interior strength is good... what exactly is it. No clue.
It's a gauge of how much work you have done upgrading tiles with civilian units. I'm not sure exactly how it's calculated, but you do get a huge boost from growing villages into towns and towns into cities.
Our exports are in danger. Or can I buy and sell overland? Don't remember.
No you cannot. All trade is ocean-based, moving directly from capital port to capital port.
I ask myself if there is a penalty for Britain because they declared war on us.
Yes there is, but you won't see it in the status screen. The diplomatic ramifications can be seen using the informational tabs in the Diplomacy screen (the buttons in the middle of the box right below the world map). You can check to verify that indeed, Great Britain suffers from declaring war. Unfortunately, they declare war in ways that prevents them from losing minor nations, and we all know that other great powers would sooner declare war on the player than declare war on a strong AI opponent.
Industrially we are well developed. I didn't know this. I thought our industry is pathetic and quite small. Obviously the other nations are doing even worse.
Industrial strength doesn't look at actual output, only the maximum amount of work the factories can support. There are several good reasons in Imp1 for you to upgrade these factories as much as possible, even if you don't have the labor/resources to run them anywhere near full capacity.
Basically, when you have spare lumber/steel/fabric, you always want to be putting it into merchant marine, transport, or factory upgrades. Whenever you can afford to not sell furniture or clothing, you should be making workers at the capital. Whenever you don't need to be bringing in other supplies, bring in more food so you can support more workers. Basically, take every opportunity to increase everywhere your industry can allow, even if you won't be making use of it. It's far better to have unused transport, labor, or factory capacity, than it is to have ungathered or unused resources.
Maybe show them delayed (like the values are from two turns before) to not give too much knowledge away, especially in multi-player mode.
I'm gonna suggest that it isn't necessary to delay them, because these stats aren't the complete picture. We don't see how much money a nation has, we don't know how much of each specific resource is gathered or exactly what is produced. We can look at the labor ranks and guess at what nations might be able to mobilize into military units, but then again they may not be producing cannon or making enough money to do so.
I think Imp1 did the status screen very right.