This post is very misleading: DL GMs have lounge access (DL and all SkyTeam partner lounges, but not contract lounges) when flying SkyTeam international itineraries (with a few exceptions). DL DMs get SC memberships. The only lounges where PM makes a difference are the VS operated ones if the OP is in coach.

If OP's employer uses DL and OP is being sent overseas, OP should ask whether the employer can arrange a status match with DL. At SEA, DL has been very generous with status challenges and extra MQM promos, so even without the employer arranged match, it would be easy for the OP to get and keep a reasonably high status level on DL for the duration of the London assignment. It's often best to have status on the carrier you actually fly as many elite FF benefits go only to the airline's own elites rather than partner elites who at best are at the bottom of the list for upgrades, etc.

Note that DL's rules regarding MQMs are NOT changing, although there may have been some adjustments in the charts for partner earnings.