I have a question about joint bank accounts and citizenship.
Now, my bank will let her joint account with me; but does anyone know about UK banks (her's is NatWest and I'm waiting on their reply) that do that?
Yes, you can hold joint bank accounts in the UK. However, it's unlikely that she will be able to add you to her account until you are living in the UK on a spousal visa... because you will have to go into the bank with her in person, and show them your passport, valid visa and a utility bill with your UK address on it.
Some people have found it very difficult to set up a UK joint account even after they have moved to the UK - it has taken several months in some cases, and some people found it was actually easier for the US citizen to open their own account than it was to be added to their spouse's account.
Why are you so interested in setting up a joint account anyway?
It's not required for a spousal visas and won't help your application in any way. If you're using savings for the visa, as long as the money in the account is in either your name or her name (and not anyone else's name), that's all you need. You just include bank statements from whichever accounts have the money in them.
In the case of the UK citizen's income, it makes no difference if the money goes into a joint account or not - the entire income has to be earned by the UK citizen.