Lestrade's holiday was a last ditch attempt to save his dying marriage (because just LOOK at that ring tanline. He was obviously wearing his wedding ring while on holiday, took it off when he got back.) He's ok with the fact the marriage is over because, if he's brutally honest, he's already started to move on from his wife. They have been having trouble for months after all.
And Mycroft HAS made it perfectly clear he's interested...