Only difference one is free the other isn't.

You'd paying to have some hand-holding and avoid much waiting, not better or more accurate results.

Sex workers are a high priority group for NHS services - if you say roughly where you are, it's likely that there's a specialist clinic near by.

Over the last 9 months I have used a mix of online private (Fettle), outreach postal, as well as local GUM. The private online (postal) filled in the gaps when the local could not provide or was limiting the type of testing (for example no oral swabs, and at one point no bloods).

My area is not covered by free online, Fettle is an offshoot of the free SH:24 for those not covered. I found Fettle excellent, very quick turnaround, clear instructions, all labels provided. Very clear. The full range of tests cost around £150+ I did a bloods for syphilis then oral swabs which were nowhere near as expensive (£23 or so for each).

I had to have a private test at Marie Stopes before having an IUD fitted, and the only differences versus an NHS clinic were a). I got a 2-3 page document with all the lab results emailed to me, and b). they tested for some additional species, like Mycoplasma and Ureoplasma.

I use SH24 it's free online and really quick too.