This is very basic info - I am not a tax professional, and anything I say is subject to correction.
Unless your wife has US source income, or is a citizen or green card holder, your best option is married filing separately (of that I AM sure).
If you work as a self-employed subcontractor once in the UK, you will need to register as self-employed, and you will pay NI contributions and tax on your earnings. You will need a National Insurance number, and when you register with HMRC as self-employed, you will also get UTR (your taxpayer number here in the UK).
You need to read up a bit on a few things. For example, in the US you would typically file a Schedule C as self-employed, and also pay SE tax. Because of the 'Totalization Agreement', you will no longer be liable for SE tax in the US when you pay NI contributions in the UK.
The first year (2018 for you) is the most complicated. After you have been resident here for the required time period, you can take the Federal Tax Credit or the FEIE (whichever is appropriate) to avoid double taxation.
Once resident here, you do get an automatic extension to file your US tax return, but any tax due must still be paid by the normal April 15th deadline.
FBAR only comes into play when you have signature authority on a foreign account or have foreign financial accounts in your name. That's an on-line only filing for the Treasury Department, and not part of your 1040. It's simple to do, and depending on your circumstances, may not be necessary. It only comes into play if the aggregate value of all accounts reaches $10,000 at any point during the year. You might need to file for 2018 if you transfer a lot of $ to the UK, for example.
Read now while you have the time, formulate questions, and post. Someone with WAY more knowledge than I ever will have will come along and provide answers.