JetBlue will never operate from LHR for obvious reasons and it's usually the case when ever a new airline intends to serve London that LHR is always mentioned.
JetBlue have apparently been looking at various options and like any other airline they will be in contact with several airports to see what sort of deal they can make or what suits there needs.
By all account JetBlue have or are supposed to have been in contact with BHX, GLA, LGW, LTN, MAN, STN so there is absolutely no guarantee that LTN is at the top of the list and why should it be especially as either LGW or STN would be a better option for them.