Promised to report back on my experience with NWP. I also ended up having to use Collinson.
I used NWP for DAY 2/8 and Test to release day 5. Test to fly home.
NWP packages came in a timely manner. As you have to mail them back it really depends on when the post gets collected. Once the package gets to NWP it is less than 24 hours to results.
Test to return package took awhile to get to NWP so i booked a Collinson in person test at St. Pancras. Results took over 96 hrs to return. Collinson promised results within 48 hours. No refund has been processed as yet so have initiated a charge back.
Rapid PCR at heathrow from Collinson results came back within an hour.