Thanks in advance.

There is almost no chance you have syphilis, especially if you use the term "escort" in the usual sense:  expensive female sex workers by appointment, in contrast to brothel workers, bar pick-ups, etc. Such women have very low rates of HIV and transmissible STIs:  most are aware of the risks, protect their health (condoms etc), get tested frequently, and their clients generally are low risk for STI (men like you). While this doesn't mean she is at no risk, I don't recall that I have ever seen such a person who had syphilis. And you are correct about the low frequency of syphilis in women in the UK.

That said, chancres can be variable in appearance; a painless ulcer at a site of sexual contact should be taken seriously. I doubt it's syphilis but cannot guarantee it. Only about half of people with syphilitic chancres have positive blood tests. However, you don't need to wait another three weeks; yes "a test could be taken sooner". Probably everyone has a positive blood test within 10 days of chancre onset. You can expect another negative result if you retest at that time.

Herpes is another consideration. While your description isn't typical for newly acquired oral herpes, in general herpes is the most common cause of genital ulceration (and many oral ulcers after appropriate contact).

Superficial social kissing probably never transmits syphilis or herpes. In general, both HSV and Treponema pallidum (the cause of syphilis) must be massaged into the tissues for infection to take hold. That's why chancres and initial herpes symptoms typically are at sites of friction during sex, like the penile shaft, vaginal opening, or labia. 

You might consider not diagnosing yourself, but visiting one of your country's excellent NHS GUM clinics, where you are pretty much guaranteed top quality care. Maybe not required in your case, but it likely would be a reassuring visit.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD