I had a similar experience as you described. I pushed it heavily up the chain at United including writing Oscar and, repeatedly got turned down with the exact language they gave you. In this most recent case which was United delayed 4 hours flying TXL-EWR, I ultimately pursued it through the German Transportation Board (I forget their exact name but they provide mediation in travel disputes like this in Germany.) This finally got United's attention (all airlines that fly to Germany are a member of this group which is funded by member fees. United bargained down to half the standard compensation 150 Euros per pax (not 300). I took it because there was no cost to me and I was tired of dealing with this compensation request. Taking United to court in Germany would've been too expensive and difficult for me to do from the USA. The mediation was free and took about 6 months. With Brexit upon us though, your options may be limited as EU 261 will no longer apply for flights departing from LHR.