you will not get into a hospital in the Uk without it.
I did yesterday. No sign of any monitoring at Leicester Royal Infirmary.
Ours were removed once processes and PPE were in place.
You can wander into the main concourse of the hospital with a face mask on.
If you're going to a clinic (and in the case of that it's only if the clinician believes they have to see you not speak on phone/video) I think that's all that's needed too along with a self assessment, the Aircon is off anyway from what I noticed last time I was down...
If you're being admitted then you'll have been sent for a test at an NHS run site, where an actual NHS staff member will assault* you with a swab, because HCPs don't trust people to assault themselves approrpiately.
A colleagues been working on the testing site at weekends, mostly doing phone calls, explaining to people that if they won't take the test then their 18month RTT is reset is apparently a bit stressful because the sort of people refusing tests are usually primed for being dicks already although some say they've had a test already somewhere else... which means they have to be told the self-swab isn't reliable...
*(there are lots of healthcare processes that without consent would be covered by Assault here)