IMHO, I would use your UK address for filing both the 1040 and the CA part year resident returns for 2014. Not to do so might imply that you are not resident in the UK, which you now are, since a fiance visa is the first step to settlement.
You are entitled to use the NHS, which I believe negates any requirement to have insurance in the US under the ACA for 2015, and as you stated, you were covered in 2014.
If you have little/no income in future, MFS is typically the best filing status. There's no reason to expose your (future) spouse's income to US tax if your liability is nil, which it likely will be.