Welcome to the Forum.  As you probably know but are trying to work around, clients on this site are not permitted to request who responds to their questions.  All questions regarding herpes go to Ms. Warren and Dr. Handsfield and I split all other questions.  As it happened, today I happened to pick up your question.  As an FYI, having worked closely for more than 35 years, Dr. Handsfield and I never disagree on the content of our replies although our verbal styles vary.  I will be addressing this question. 

The initial symptoms you describe, urethral pain, going into your urethra, are certainly compatible with urethritis, either NGU or even gonorrhea although gonorrhea would have most likely produced a visible discharge.  The rapid progression to testicular pain without a discharge and pain going into your thighs however is not suggestive of an STI.  Your negative tests appear to confirm that this was not an STI.  Irrespective, the treatment you received, azithromycin, flagyl, and what was almost certainly ceftriaxone would have cured nearly all cases of gonorrhea and NGU and the rapid response to therapy seems likewise to support the diagnosis of an STI.   Then, things get a bit more complicated.  Your recurrent symptoms after re-exposure to your then regular partner support the possibility that you may have acquired something from her.  Whether this was an STI such as non-chlamydial NGU in which her normal vaginal bacteria entered your urethra or something more sinister such an STI which you reacquired on re-exposure is difficult to say. 

That you have inquired and that you are thinking of informing your former partner suggest that you want to do the right thing.  Further, I presume you want to know if you might have an STI that was missed earlier.  For both of these reasons, I would suggest that you do tell your former partner of your symptoms, that you were treated (no need to mention London) for them and that this improved your symptoms and that you wanted to let her know.  I would also tell her that you were tested and that all tests were negative and that your doctor (or me) has suggested you inform her despite your negative tests, just in case.  I would also ask her to let you know if any of her tests are positive.  This is both the right thing to do and will provide you with useful information.

 I should add that in the UK, many experts do not recommend partner treatment or notification for non-chlamydial NGU but my sense is that until things are better sorted out, probably bet to notify. 

This answer is rather nuanced.  I hope this comment is helpful. Please let me know if anything I have said is unclear or if there are additional questions.  EWH

Thanks for the clarification.  Testing at this time would certainly be reasonable and, I suspect, helpful to you.  I am confident that the tests will, once again be negative. 

Your are correct, there is no indication that most non-chlamydial NGU is not harmful to sexual partners and does not lead to complications or other problems.  As I mentioned above, in the UK, partners of persons with non-chlamydial NGU are typically not treated.  There should be no untoward problems for her if you do not tell her of your concerns.  EWH

Correct- there is no indication that partners to persons with non-chlamydial NGU are at risk for problems, including complications of infection. 

I am confident your tests will be negative. 

There is no reason from what you have said to worry that your ex will cause infection in her subsequent partners.  this is not something that it is useful for you to worry about. 

As you probably know, Forum Guidelines limit us to 3 replies to each question.  This is your 3rd reply.  There will be no further replies and the thread will be closed later today.  EWH