Can you get a multi-decade fixed rate mortgage in the US? I don't think that they are obtainable in the UK, most fixes only cover the first couple of years. Is it a two way commitment so that you are locked in for that time period and it makes it harder to move/downsize or can you pay it off early?

Yes you can. The product most people get these days is the 30-year fixed rate. 15- and 10- year are also available; as you shorten the term, the APR decreases. They typically don't have any pre-payment penalty.