Lots of mentions of TravelUp on the BA forum, just search for "travelup". Here are a couple of threads:
https://www.flyertalk.com/forum/british-airways-executive-club/1973671-always-cheaper-business-class-fares-through-3rd-parties.html
https://www.flyertalk.com/forum/british-airways-executive-club/1888094-ba-booking-through-travelup-co-uk.html