You might want to keep your US bank account open, even after you leave, if you can feasibly do so (if there aren't fees that would just gobble it up). Once you're moved, it's almost impossible to get a bank account in the USA. And they are really handy to have - direct deposit of tax refunds, paying US bills, stuff like that. It's also nice to have US credit cards, especially if they have no foreign transaction fees.
As noted above, don't inform your employer that you "might be" or are "expecting to" leave. When you have approval, that's soon enough. Traditionally one gives two week notices, when possible. If not possible, as much as you can.
Transferwise is easy to set up and easy to move money over with. Once you have a UK bank account, you can do the transfers online. In either direction. They give you good rates, and you have a choice of several different options for how you have the money sent.
Almost there, you'll be on the way soon!
