This deceptive and uniquely coronavirus pitfall has been discussed here before following an articles posted by Shyaku on 22nd April. It's now getting a little coverage; the Sun has covered it but haven't seen it elsewhere in the UK mainstream. The link blow is a local news outlet.
Derek suggested a doctor or clinic should be able to do this - but I think the problem is, they have a rather fixed view of what it means to be a 'sick patient', so it might not be easy to get an appointment if they think you are not showing severe symptoms.
If you have access to a pulse oximeter gadget it can tell you when your sats are low. The were available in places like Argos and Amazon but I think they are mostly bought up now.
In my view something needs to be added to policy to do something about this as it seems to be costing lives, for want of something quite simple. It seems to me the facility should be made available to everyone with Covid-19 symptoms.