I have done it a number of times, and never had a serious problem. I use a Frio wallet to keep it at a stable temperature.
Security at Heathrow don't like you to have more than one hypodermic needle, but I have always argued to keep two in case I drop one. The rest I put in my check in luggage. They have also tried to insist that the cabin crew look after the insulin, which I have also politely and sucessfully resisted on the grounds I might need it immediately (!) and if their life depended on something would they want to lose physical control of it.
The main problem I've had in the US on domestic flights is ignorance (check it in, inject in front of me, what is a pen etc). There has always been someone (normally a supervisor) who has more knowledge so a polite request to talk to someone with experience has worked.
On a practical level, make sure you test more regularly. During and immediately after the flight. I usually let my blood sugar go a little higher than normal as a precaution. I am on DAPHNE, so adjust very quickly to a new time zone (shame the sleep patterns take longer). My dad, who is on a more rigid program, takes longer (2-3 days) to get his readings to normal levels.
Oh, and US food is typically loaded with carbs and have portions the size of small family cars, so mind what you eat.
Have a great time, and don't worry, it will be fine. I have done over 200 long hauls with insulin and never had a problem that a polite explanation could not solve.