What should I do if my METV application is still pending approval before my flight to Thailand?

My DTV was issued the day before I was flying. They do take note of your flight date and prioritise accordingly.

My german friend was waiting many days for his metv too

That's plenty of time to get your visa. London is usually pretty good.

Worst case scenario is you fly without the visa and hope it comes while you're traveling, or enter visa exempt and just use the visa next time. Unless you have a long history in Thailand this year, and you're worried you won't get in without the visa.

If that's the case, you can use a safe entry agent to get in this time, and then try the visa the next times. That is probably cheaper than changing a flight.