As Guya states above and I agree, I include UK employer pension contributions in addition to my gross (it usually doesn't have any US tax impact in the year of filing) but (I understand) this protects you further when you eventually come to draw down or receive pensions. I also take a belt and braces approach and claim a US/UK Treaty position - the jury is very much out on whether that has any benefit!.
In terms of tax credits, you can claim UK Income Tax in addition to any UK Capital Gains Tax that you may have paid - you cannot claim UK National Insurance, which is the UK equivalent of US Social Security.