I have a doctor prescribed oxygen saturator. The proper medical ones cost at least 2000 dollars. Im not sure I would trust one that cost 300 pounds. If you do use one without medical supervision (which I wouldnt recommend) you absolutely MUST also use a pulse oximeter so that you can tell when your saturation gets back up to 99% and can adjust the flow of the concentrator to a lower setting. It would be dangerous to over oxygenate iyswim.
If you definitely want one, look for one that has a pulse setting, they only push the oxygen when you take a breath. Also try and find one that has lots of levels, so you can turn up or down the flow.
I have tried a Phillips Simply Go Mini and an Inogen G4. Both are very good and very portable.
But again, this is serious stuff to use without the help of an experienced doctor.
For pulse oximeters, a 20 pound one on Amazon will be fine (look for one that takes standard batteries as some need the annoying to find button ones), just always try a couple of different fingers if you are getting a low reading, sometimes you can have muck on your finger and not realise it and it gives you an artificially low reading. I agree that anything in the 97% plus area is completely fine, below 97 and above 94 is cause to watch carefully and talk to a GP (they will want to know if you can get the % to increase if you do deep breathing - as that is a good sign). Below 94% is very worrying and 90% or below is get yourself to good medical care sharpish.
Hope this is helpful, if anyone wants to chat more, then feel free to pm me