My deal with my employer is that I get an official, policy-compliant quote from our agent, and as long as I can beat that, I can book myself. I have to achieve equivalent fare flexibility as well, so if it’s a fully flex quote then I have to book the same.
Since I have clients all over the world, who will generally meet me whenever I want, I find a cheap ex Eu fare, fix up some meetings wherever in Europe pre the longhaul, and everyone is happy. Ex Eu flexible fares are an unbelievable bargain, typically 3-5k cheaper than ex-Lon, and since they are fully flex, I can keep moving the last leg around till I need it.