I see Dean Street Express is not taking appointments. Are they are any other places in London that can do a quick test?
DSE are most certainly taking appointments. They release their next day's schedule online at 19:00. You have to hit the system then. Depending on the day they fill up quickly especially the 0800 and lunch slots. Pro tip take the 0815 rather than 0800. Miss the queue and you can usually slide right in.
They should be clearer about the 19:00 thing on their website.