Wizzair with £2 tickets, how do they make money?

Wizzair’s ancillary sales of 1.2 billion Euro accounted for 45% of total revenues(2017).

Ancillary services are considered those beyond the transportation of the passenger from origin to destination. Airlines provide various services and engage in other activities connected with its core air passenger service, including non-flight scheduled services, internet-related services, and the in-flight sale of beverages, food, and merchandise.

Because they're competing with other low cost airlines, not the underground.

Why not? They could just charge more for other costs (luggage, seat allocation, etc) since those are not disclosed upfront.

Average revenue per passenger improved by 5.2 per cent from €69.0 in F20 to €72.5 in F21. Average ticket revenue per passenger decline from €37.7 in F20 to €32.0 in F21 (by 15.2 per cent), while average ancillary revenue per passenger increased to €40.6 from €31.3 (by 29.6 per cent)

Wizz air annual report.

Lots of people spending a little adds up. Poundland for example, small amounts of revenue for each individual product, but millions of transactions add up.