My suggestion is to confidently hand over your passport, already opened to the pretty visa vignette, and give a pleasant smile.
And your landing card! Don't think just because you have a visa you don't need the landing card...
It usually goes like this:
IO: "How long are you visiting for?"
You: "Forever" (usually with giddy exhuberation)
(IO, Actually looks at Visa)
IO: "Where is your fiancé?"
You: "Waiting for me [insert wherever]."
IO: "Welcome to the UK"
*stamp* *stamp*