When I went through Bordon (around 4 years ago) the VM's were told to choose which they wanted, but not everyone would get their choice, as the majority want to go A Mech and there just isnt the spaces avaliable. Cant imagine it has changed much, just put down your choice and then wait and see what happens. You may aswell get used to being told to do something you dont want to do anyway, your joining the army.
I was down as A mech all the way thru ATR bassingbrn, got to bordon and the first 5 on the register got TOLD they where being A mechs, that was 15 months ago.... now i am a class 3 vm B, and i stand up to piss when i am sober, and lye down when im pissed! as long as you arent a rekimek or a warhammer geek its all good right? i am a big boy arent i?
Also... im sure when you ask someone who is supposed to know about theese things, they will tell you there will be no vm A or vm B soon as it is all changing to vm-U .. U for universal, or Unisex if you like. i think that has been a rumour since black and white tv, and hasnt gone away!