I've just had a pulse oximeter delivered (not from Q though, and less than £20 with free pp).
In one part of the UK, there is a trial for everyone to be given a free one to monitor blood oxygen.
They have discovered that, if you have non-symptomatic Covid, your oxygen level can fall dangerously low (when you would normally be dead) but you don't have any outward symptoms and seem OK, until you drop dead if it goes any lower.
Last time mine was tested (at the Dr's) it was 98% which is fine.
It will also monitor my pulse when I am using my gym equipment.