The exposures you describe are low risk.  Most CSWs do not have STIs and that is particularly true for "high end" CSWs.  Further, receipt of oral sex is a particularly low risk exposure.  HIV is virtually never acquired by receipt of oral sex.  There are NO proven cases of HIV acquired from receipt of oral sex.  Further, acquistion of chlamydia and syphilis are very uncommon.  Of the STIs which may be acquired from oral sex, gonorrhea is most common and when gonorrhea occurs, it is typically obviously symptomatic.   Depending on how long ago you last exposure  was, there is little risk of infection.  

If I were you and it has been more than 3-5 days since your exposure, I would not worry and would not bother to test.  If you are concerned/anxious, testing is easy.  If you choose to test, testing for gonorrhea and chlamydia (typically both tests are done at the same time) is easy, involves a simple urine test and is highly reliable.  If you test at all, that is the only testing I would bother to do.  EWH

1. Don’t worry, very unlikely I caught anything

2. If no symptoms after 3-5 days no need to test (what should I look for)

3. I am okay to continue my relationship if we move back in together with my wife

Can I ask why the messaging from the NHS and government would lead me to believe it is a certainty I would catch something? How do I find the evidence to put my mind at ease?

2.  Correct.  The symptoms of the most common STIs are a penile discharge or burning during urination.

3.  Correct 

The role of the NHS is to encourage testing.  In addition, they tend to be conservative.

The only way to be 100% sure is to test.  In my opinion, there is a more than 98% chance that if you test, the tests will be negative.

Please don’t worry.  EWH