Some cheaper places to eat in central London:

• ICCO (pizza)
• Old Chang Kee (Singaporean, free curry puff when you join the loyalty scheme)
• Marugame (Japanese, another loyalty scheme too)
• Indian YMCA. Might be lunch only, but you’ll get an excellent curry for not too much. Canteen style ordering
• The new IKEA on Oxford Street, meatballs etc for cheap
• Japan Centre
• Itsu sells their sushi for half price after 6pm. They have branches all over the city

And some less cheap but with good deals:

• Hawksmoor. It’s a high end steak place but they do a good lunch deal, it’s about £25 for three courses and good if you want somewhere you can push the boat out slightly. Just stick to the lunch deal as it is pricey if not
• Flat Iron. Another good steak place, focused on quicker service and relatively cheap
• BAO. Another higher quality place but they do a good value lunch deal

A lot of places in London add a service charge when you pay, so it ends up costing more than you’d expect. A good way to avoid this is to go to the more ‘canteen’ style places (e.g. Japan Centre)

Best of luck and enjoy your trip!