I was an MFL teacher until I had ds. I taught German and Russian. I would call my level of fluency 'near native' in German - I grew up, went to secondary school there, sound German, get taken for German, but don't visit often enough to keep up with current slang. I learnt Russian for 9 years, spent two years living in Moscow and still struggle with it. The 'native sounding' people on my degree course went native and lived with Russian boyfriends or girlfriends. I only ever managed to be taken for someone from one of the Baltic states.
As a teacher, it is very difficult to get pupils to a high level of fluency as only the most motivated will put in the hours outside the classroom which are required. For A-Level, a student should be doing self-study (surfing the web, reading magazines, watching films, learning vocab. etc) and homework. We reckoned that one hour of teaching required at least one hour of independent study to get an 'A'. It is always obvious who puts in the hours.
I trained ten years ago, and we were required to teach in the target language all the time. Of course, many MFL teachers are not subject specialists.
Regarding the OP, Spanish GCSE entries have just overtaken German entries. French will be dominant for a while yet, as it is a chicken and egg situation regarding teachers. More children learn French at school, so more people study French at university and become teachers, so more French teachers are available, so timetabling is easier, so French is offered. Parents also, ime want their children to have the option of learning French. It is a big financial committment for a school to offer a new language on the timetable. Unfortunately there a lot of places that money needs to be spent in a school and MFL is frequently not a priority.