Thank you! Mostly I'm concerned about the passports and the marriage license, but I'll comb through to double check that there's nothing else I'm interested in requesting back. Thank you again!
Well, they aren't going to keep your passports, because that would be illegal (your passports are the property of the US government and the UK government is not legally allowed to keep them).
You shouldn't send your husband's actual UK passport, but just a certified copy of his passport photo page.
The marriage certificate should be returned to you, but if it isn't you can always contact them to complain and request it back (you should also be able to get more copies from the county where you got married if you need them in the future).
Will your husband want any of his original documents back too? Things like bank statements, payslips, job offer/contract, your in-laws' land registry document, mortgage statements etc.
I would send photocopies of basically everything you are sending, just in case.