This is a pretty high risk encounter, objectively. Obviously, if it's truly been some time since her last partner, the risk goes down some, but she can also have STDs and have no symptoms.
I know in the UK, they recommend waiting longer than they do here in the US. I don't know why, and those were in place pre-pandemic. In the UK, it's 2 weeks for gonorrhea and chlamydia. In the US, it's 3-5 days.
For others, it's similar. Syphilis is 6 weeks. Syphilis is uncommon in most developed countries, and it shouldn't be your first concern.
HIV is 28 days with a 4th gen DUO test. Most people don't have HIV, so while you should definitely test, I wouldn't overly worry about this.
If she has herpes type 2, there is a chance you could get that. You can test with a type specific IgG blood test at 12 weeks. It's not easy to get those in the UK, but if you don't get symptoms within 2-12 days-ish, I wouldn't worry.
You can test for mycoplasma, another bacterial STD, that gives you similar symptoms to gonorrhea and syphilis, when you test for gonorrhea and chlamydia.
There is also HPV, which has no test for men, and you might not get symptoms for months, if you get symptoms.
Unfortunately, your partner is 21, putting her in the highest risk age group for STDs. Since we don't know her history, we don't know how high risk she specifically is. You not ejaculating doesn't mean much here. It might protect her if you had something, but it doesn't help you much, if any.
I can't advise you on what to do with your wife. If it were me, I'd at least use condoms, and avoid sex if I got symptoms. Testing is possible in the UK, though it may not be as convenient as it used to be.
Good luck, and let us know what happens.