About getting an STI screening...

I just started living in a new city (in the UK) and have my own place so being the genius I am I decided one of the first things I'd do was try a random hook-up online. I enjoyed it but despite the guy saying he had condoms, I don't think he actually used one in the end so I remained as calm as possible and tried to contact the guy to ask him about it but he hasn't replied since. I figured I should also go for an STI screening but I was wondering when the optimal time to do it is? I've heard some STIs are undetectable at first so I'm not sure if I should wait for some time or go straight away. Can anyone give me advice?