British library obvious choice but local branches like holborn, westminster reference library, kensington central library have fast wifi, power outlets, printing facilities without the st pancras crowds.

Stay all day without pressure of buying coffee every hour at pret. Westminster reference library on st martin's street sits 2 mins from trafalgar square with upper floors surprisingly peaceful for laptop work.

Perfect when you need genuine quiet time between westminster abbey and national gallery. Free visitor passes, just bring photo ID to any library.