I'm feeling like I've exhausted most permutations of lamp types in these lights. I needed to go into Halfords today, and somehow came away with a pair of Advanced White 5000 halogen bulbs. These are my findings so far.
Phillips X-treme Ultinon LED. Reasonable light output, but way off OEM LEDs. Supposed to be 5800K, but had quite a bluish tint. Maybe I got a bad set, but intermittent flicker from both sides. I tried swapping the resistors from side to side, but it couldn't be cured.
Twenty20 LED. These are said to be 6000K, but a much whiter white than the Phillips. They look very much like OEM. However, the light output is extremely poor. I'd go as far as say dangerous. These are the 150% brighter ones. They do a brighter one, so maybe they would be better. I also suffered DAB problems with these. I was struggling with Radio 2. As soon as the lights were turned off, the radio came straight back.
Halfords Advanced White 5000 halogen. These are said to be 5000K, but not a chance. These are quite white for halogens, but still look pretty yellow compared to the DRLs and the Osram Cool Blue Intense main beam bulbs. How they perform will be reported back once I've been out after it gets dark.