UK Bound (and Determined!)

Hello, welcome and best of luck to you! Congratulations on your upcoming wedding!

I don't have any info for you on the immigration front, but I can say that when my then fiance came to the US so we could get married, he had his return ticket handy but we  would also strongly suggest your fiance does not mention he is in the US to get married. That raises their suspicions. Even if you have a return ticket they suspect you are going to try to stay once you let them know you're here to get married. Just a tip. 

One shouldn't lie to the IOs, either directly or by omission. It is perfectly acceptable to get married in the US on visa waiver and tons of people on this site have done so.

I don't have any info for you on the immigration front, but I can say that when my then fiance came to the US so we could get married, he had his return ticket handy but we  would also strongly suggest your fiance does not mention he is in the US to get married. That raises their suspicions. Even if you have a return ticket they suspect you are going to try to stay once you let them know you're here to get married. Just a tip. 

No, definitely don't lie. If they ask why you are in the US, you should mention that you are there to marry.

It's also a good idea to print off this page from the US Embassy website, which explicitly says that if you want to marry in the US, but not live in the US afterwards, you should travel and marry on the visa waiver program (or get a visitor visa if not eligible for the visa waiver): http://london.usembassy.gov/immigrant-visas/marriage-to-a-u.s.-citizen.html

From that page:

Fiance(e) Visas

If you will marry in the United States with the intention of taking up  indefinite residence after marriage, you will require a fiancé(e) visa.  For further information, please follow this link.

Note:  If you will do not intend taking up indefinite residence in the United States but will continue to live and work abroad after the marriage ceremony you should apply for a B-2 visa, or if eligible travel visa free under the Visa Waiver Program.