You shouldn't have 3 fleet types to begin with--you only have 12 planes (four 727, four 737, four DC-10). If I were you, I'd focus on ditching the 727 for 737 and worry about going to MD-80s later in the game. MD-80 has better fuel efficiency, but the improvement over the 737 isn't enough to justify stunting your growth to wait around for MD-80s to get built. You should focus on getting more 737 to expand, which are much easier to come by on the used market than MD-80s. Once you have a sizeable airline with only 2 fleet types (737, DC-10) you can transition to the MD-80 much easier. Heck, the MD-88 may even be out by then which has much better fuel efficiency than the MD-81.
You're based at LHR also, which means you may want to consider going with the 757 when it comes time to upgrade since slots won't be available forever.