National Insurance vs. Social Security & Other Questions...

I may be way off base here, but it seems like I read somewhere that you have the option of making social security contributions in lieu of paying national insurance contributions.  Is this at all true, and if so - what consequences are there, if any, of choosing to do this?

Secondly, I realize that the US & UK have some sort of agreement regarding the contributions made to these plans, but practically, how does this play out?  If I pay NI for 2 years, do I get a UK pension, plus a US Social Security check?

Thanks in advance...

No. 

The US and the UK have a Totalization Agreement that covers all NI/SS issues.

If you came to the UK working for a US employer and opted to remain on the US payroll, then you could have had SS withheld instead of NI.  Otherwise, it's NI with the added bonus of no obligation to ALSO pay into SS. 

When you retire, you will get a prorata UK pension for the number of years you worked.  The US SS will also be prorata.  Thus, your little statements showing how much SS you will get are overstated, since it will be prorated for your offshore working years.

Liz is 100% correct, if you want to read further the details can be found here:

http://www.ssa.gov/international/Agreement_Pamphlets/uk.html