MrStrain14

Yes, you can, but it is upto the immigration officer at POE as to whether you will be allowed into the US.
Take evidence with you that you have intent to return to the UK - a letter from your employer is good to start with, recent mortgage statement, return plane ticket and anything else that ties you to the UK. Also a copy of the NOA2 & Embassy packet 3 letter to show you are applying for a K1, shows you're not trying to hide anything.

For the London Embassy Interview, you will need the I-134. Once you apply for AOS (adjustment of Status) in the US, you will need the I-864.
A guide to completing the I-134 can be found at http://www.visajourney.com/content/support, or by searching a bit more on the London Thread for I-134 related posts. There is plenty of information on here, use the search tool at the top of the page.
Good luck!