Check in with the UA Card - then you should get the centre seat block if in Y and you'll be somewhere on the list if they need op-ups (behind bmi *G and *S). You can change to the bmi number on entry to the lounge.

Otherwise, use the bmi number at check and flash the card to use the lounge - NB *G is upstairs and the lounge matron will call upstairs to have the doors opened for you.

Another thought - check-in is likely to be a scrum. Use whatever you can to check-in at a kiosk, or use a business desk if there's no queue. You can sort out any numbers and/or seat blocks at the lounge.