I have a 4.5 hr connection in Heathrow Terminal 5 (US Passport holder) and would like to know if there is kosher food available for sale in the airport, i would go to a diffrent terminal if thats where its available
According to Kosher Today, at least as of 2015, there are kosher sandwiches and wraps at the WHSmith stores in terminals 2, 3, 4, and 5. There's a phone number on that page, and it might be helpful to call ahead and confirm.
Failing that, The Heathrow website says that kosher food is available at Caffe Nero. In Terminal 5, that restaurant is located in the arrivals area, meaning that you would have to go through UK Immigration to access it. There are also locations post-security in other terminals, but you cannot simply just go there unless you have a flight from those terminals. They will allow you to change terminals for expensive shopping trips, and it's possible that they could offer you the same service if you inquire in advance, should you not want to cross the UK border. However, WHSmith in T5 is post-security and likely your best bet.
The website of Hermolis, the kosher caterer they use, confirms the above.
There are also reports from a few years ago that they keep a supply of frozen kosher meals on hand at the British Airways lounge, should you have access to that facility. That forum thread has some anecdotal experience that would be useful to read.
Since these meals are brought in prepackaged by an outside company, it would be a good idea to bring something with you, in case they are unavailable.