I am a UK citizen living and working in the US. When I return to the UK can I get a credit for the social security taxes I paid over many years in the US - in other words does reciprocity exist on social security (FICA) taxes paid between the US and the UK?
There's such an agreement. It is called "Totalization Agreement", you can find all about it on the SSA web site.
You'll have to read through it and talk to a licensed tax adviser in the UK to know exactly what it entitles you to.