The matinee on the 20th is sold out but all other performances around the rough date you've indicated are showing 'limited availability':
https://www.birminghamhippodrome.com/calendar/les-miserables/#performances
Taking the evening performance on the 20th as an example, I'm seeing just two seats (together) available in the front stalls (at £76 each), 1 seat in the front circle (£76), 4 separate seats in the rear circle (£69 or £76) and 1 seat next to the mixer desk in the rear stalls for £36, with the boxes being sold out.
Clicking on an available seat brings up the option to click on 'Standard', which adds it to the basket. So where does it go wrong for you?