The savings clause (article 1.4) says the US can ignore the entire treaty when it comes to US citizens. So it does. There are some exceptions, found in article 1.5.
Pensions are not 'earned income'. The $100,000+ tax exclusion as found on form 2555 (FEIE) is for earned income only. You would use FTCs (Foreign Tax Credits). They comprise of the tax you pay in UK, but not all the tax you pay in the UK. If the US does not have an equal type tax, tax paid in the UK won't count.
What is tax free in the UK is rarely tax free in the US. Think of ISAs for example.
Some types of UK investments can be very toxic for a US tax payer. Think of Stocks and Shares ISAs, or any type of mutual fund.
The filing threshold for a filer of a 'married separate' return is $4,050 for 2016. Above that figure, you must file a return.
If you are delinquent in filing for past years, there are special programmes you may qualify for to get back into compliance.