When Jet Set Willy was released for the ZX Spectrum it was buggy and impossible to complete. To fix these bugs Software Projects released these official POKEs. You should enter before playing the game:

POKE 60231,0 POKE 42183,11 POKE 59901,82 POKE 56876,4

See here for more info about what these POKEs do:

https://skoolkit.ca/disassemblies/jet_set_willy/reference/pokes.html#theOfficialSoftwareProjectsPOKEs