I’ve lost mine, but a quick summary.
We are sending you this letter because you are in a category that may benefit from
treatment for covid, ie over 80 or lmmunosupressed.
You will be sent a test kit in the next two weeks and take the test if you have symptoms.
I am sure about the over 80 bit, you are to young

Report a positive result on 119 or online, you will be told what to do
A bit of digging shows that this group has always been eligible for free tests even after they were no
longer free for most.
So no connection with vaccine, just giving us them without us having to ask.
Googling “report a positive test” is where I found this.
Mike
A quick edit, you can order a pack of seven every three days, to test your nearest and dearest as well