I went on here:
Find information on coronavirus, including guidance and support.
www.find-travel-test-provider.service.gov.uk
Sorted by price then went to Medicines Online Ltd (£1.89) - I checked the cheaper two ones but they had no availability, probably doesn’t matter as really only doing it for the reference number but wanted to make it somewhat believable.
Their website isn’t great but if you click “Covid Services” > Forward to page 2 > Day 2 Self Swab Unsupervised - Battersea.
Note: I’m not 100% on what I’m doing here so probably best to wait for someone else to confirm this will work.