Thank you for your comments.
Your encounter was protected; therefore it does not involve any risk of an STI. One cannot really contract an STI through kissing as such. Perhpas only if it was deep throat kissing, there is a risk of possibly contracting Chalmydia or Gonorrhoea. There is also a possibility of herpes if the lady had a cold sore, but that is HSV1 and not considered an STI. No real risk of syphilis. No risk of HIV infection at all whatosever.
There is no need for you to test or to worry unnecessarily. If you had a deep throat kiss, the most I would recommend you having would be a throat swab for Chlamydia or Gonorrhoea. Alternatively you could always take a pre-emptive empirical single dose of azithromycin and cefixime, which would clear these infections in 48 hours in the ulikely case that you had them.
You would have to discuss the whole situation with either your GP or your local GUM Clinic, but beware that they might not be happy to prescribe the medication as a pre-emptive measure without any tests. I would be happy to dispense them, obviously if you could come and see me at our clinic. The dose is as follows: Azithomycin 1g and Cefixime 400mg, both as a single dose.
Difficult to give advice without actually seeing and examining you. However I can say that an itchy red patch in the scrotal area sounds to be more likely something like a fungal skin infection or thrush. A genital herpes rash usually causes initially tingling, followed by pain and soreness. It has a typical blistery appearance.
What you should do is to go and seek medical advice. A doctor needs to have a look at it and give you the appropriate treatment. If in doubt, you can always have a PCR swab to diagnose herpes.