Does DD need to continue self isolating?

[Bauble1234]

DD (18) found out someone she was in close contact with yesterday has just received positive PCR test. She found out whilst travelling back from London by train. We picked her up from train station and took her straight for a PCR test. However results will take 72 hours to come . She is now in her bedroom isolated. She has had her second Covid vaccination 4 weeks ago.
Thing is, we have my 76 year old parents staying for Christmas (live 4 hours away). Both of my parents have had both Covid vaccinations and boosters.
I don't really know how to handle this. Government guidance seems to be that you only need to isolate if you test positive for Covid. We have given DD a Lateral Flow test which was negative. However as her exposure to Covid positive person was only 24 hours ago, it may be too early for LFT or PCR to pick up Covid even if DD is infected.

I assume DD probably needs to stay in her room to protect my elderly parents? I have told my parents they could go home and we won't be offended.

She doesn’t technically need to isolate. She just needs to do LFT for 7 days and carry on as normal unless one turns positive. Even though she’s had a PCR now it doesn’t mean she won’t develop Covid in the next 7 days. She might unknowingly put elderly relatives at risk if she does mix with them. Seems a bit much though to expect her the stay in her room for a week just in case.