I have some additional information for anyone else who has been registered as clinically vulnerable, that they might not be aware of.
I recently used the PCR test that I had been sent, as I had caught a cold and hence had some Covid symptoms. I sent the test in on a Thursday but did not get any feedback on the results for a few days. My symptoms reduced and as LFTs were negative and my wife, who had the same symptoms, had had 7 negative LFTs in 8 days, this gave me confidence that it was not Covid. I called the 119 number on the following Monday and was told that my test was negative and they apologised for the delay in getting back to me.
To get another PCR test I called them again and they said that the procedure had changed and that were not sending out PCR tests anymore, but were sending out batches of LFTs. With the advantage that you can conduct many tests and self register if it is positive. And the disadvantage that it is not as sensitive as a PCR and so you might identify Covid later than with PCR.
As before, within 24 hours of registering a positive test, a medical person should have got in contact and organised what would happen next in terms of antiviral drugs to reduce the severity of Covid.