@mrseffington - the information you have been given is incorrect.
That yellow box information is copied from the NHS information under:
"Rapid Tests If You Do NOT Have Symptoms"
Rapid tests if you do not have symptom
About 1 in 3 people with COVID-19 do not have symptoms but can still infect others.
Rapid tests help to check if someone has COVID-19. If people test positive and self-isolate, it helps stop the virus spreading.
Research shows rapid tests are a reliable test for COVID-19. They give a quick result and do not need to be sent to a lab.
Even if you’re vaccinated, you could still catch the virus or pass it on. Doing rapid tests helps to protect yourself and others.*
Get rapid COVID-19 tests if you do not have symptoms
Important
If you get a positive lateral flow test result, you do not need to confirm the result with a PCR test. You only need to confirm a positive lateral flow test if:
you're applying for a Test and Trace Support Payment
you're eligible for new COVID-19 treatments
you're asked to do so as part of research
you have a positive lateral flow test on or before day 2 of arriving in England from outside the UK*
OP stated she DOES have symptoms, therefore she needs to confirm with a PCR test.
This is from the same page:
If you have symptoms of COVID-19
Get a PCR test as soon as possible if you have any of these symptoms, even if mild:*
a high temperature
a new, continuous cough
a loss or change to your sense of smell or taste
Stay at home until you get your test result – only leave your home to have a test. Check if people you live with need to self-isolate
The information the poster added is from the link below, you need to read the whole page, not just take that information in isolation as it is taken from the wrong heading.
www.nhs.uk/conditions/coronavirus-covid-19/testing/get-tested-for-coronavirus/
It is confusing but if you are testing regularly for work etc and you don't have any symptoms but test postive on a LTF there is no requirement to do a PCR unless you are applying for a track and trace payment or one of the other reasons listed.
If you have symptoms, you are required to still do a PCR test.
Also, see this Govt press release from 11 January
www.gov.uk/government/news/people-with-a-positive-lateral-flow-test-no-longer-required-to-take-confirmatory-pcr-test
It also remains really important that anyone experiencing COVID-19 symptoms self-isolates and gets a PCR test via GOV.UK, or by phoning 119.