Swiss Tony, you're right. Luton and Gatwick would be the perfect choice. But I have no flights to these airports. My current choices are LH, AF, and BA. These airlines fly into Heathrow (and sometimes to City airport).
I arrived in Borehamwood in the meantime

. I had more luggage to carry so I took a cab from LHR with a special fix price (40GBP one-way = quite a good price for the distance, 65min during heavy traffic in the morning). I'm going to try the Thameslink soon. The walk to the Elstree & Borehamwood station seams to be ok (the taxi driver showed me the way).