What happens at the interview
 

Looking forward to my interview in london soon, but what exactly do they do at this interview ?

They check your documents, send you for a medical, interview you face to face (which can be as short as 2 minutes), ask you to make a sworn statement, make you pay for your visa, then finally hand you a brown envelope containing all your paperwork to hand over to the INS official in the airport.

The entire interview process takes 5 hours - from 8am till 1pm.

how much does it cost for the visa ?

It depends on whether it's a spouse or fiancee visa
The spouse one is $335 (approx £200) and the medical is £125.
Go back on here for 2/3 days and look for postings by
The Mallrat, Anonymous and janesays
They all had interviews in the last few days and explain what
happens from the time you get up until your furst pint after
the interview.
Good luck