Free BMD is transcribing death records, but death record for some years between 1870 and 1880 are incomplete and Benjamin Alexander is not among those already transcribed.
However, as his widow was at the same address in 1881, it seems likely that he died in West Derby.
Without the exact year, one cannot obtain a certificate from the GRO online: www.gro.gov.uk/gro/content/certificates
However, you may be able to order by mail
Bruce