If you are a “local hero” (teachers, social care workers, prison officers, armed forces, ‘blue light’ professionals and Max Card holders), ATG offers 25% off many of their shows. I have not seen any central London shows offered, but Richmond Theatre and Wimbledon Theatre are included. https://www.atgtickets.com/campaigns/local-heroes/featured-shows/?fbclid=IwAR1wET1W3AgvZn0Jrm6cnpd6pei-4PSFISfbJ2n-a2c3-2AAnNBP7AZQcAw
Membership of the Young Barbican scheme (for 14 – 25 year olds) at the Barbican Centre offers some deals on great shows with prices varying from £5 – £15. https://www.barbican.org.uk/join-support/young-barbican
The Royal Shakespeare Company TikTok scheme offers £10 tickets for young people 14 – 25 and full time students. Remember, it is not just Shakespeare, My Neighbour Totoro and Matilda (for example) are also RSC productions. The way you book the £10 tickets varies and requires membership first. You need to stay alert, TikTok tickets…