yes, you'll only get MFF at LGW/MAN. Although there are folk about who have moved base and maintained their MFF. Usually trainers, but this is very much the exception.
TCX will train you up on the 321, then either, you bid for MFF and wait for your turn (if you are not based at LGW/MAN you will need to bid for the base change first) or if they need you, you will be put on a course whether you like it or not. At the moment, it is expected that all new joiners at MAN will become MFF as there is a shortage to LH FO's. I don't know the situation at LGW, it may take longer as the program is smaller there.
One thing to bear in mind, TCX has changed its mind on crew planning every couple of years, so info today may not be valid in the future. If LH is a big issue for you, this may not be your airline. However, the ability to change between LH and SH during the year makes for a more interesting job. It always makes me laugh, the amount of people who have left TCX for greener pastures, only to realise later that they were actually already on one of the best gigs in the industry.