The event was hosted by the RMT union which has been at the heart of strikes that have crippled Britain's transport network in recent months as workers demand better pay and conditions to counter the impact of soaring energy price-driven inflation.
"People across the world need to stand together to tell the oligarchs they cannot have it all," Sanders told the gathering at the Trade Union Congress headquarters.
"No one can tell me from a moral perspective or from an economic perspective that it makes any sense at all that so few have so much and so many have so little."
Sanders, a democratic socialist whose progressive campaigning helped push the U.S. Democratic Party agenda sharply to the left, ran to be U.S. president in 2020 but ultimately lost out on the party nomination to eventual election winner Joe Biden.
British workers across a wide range of industries, from refuse collectors to court barristers to postal workers, have decided to strike this summer as double-digit inflation surges ahead of wage increases.
There is little end in sight to Britain's industrial unrest with the government urging restraint from employers, companies facing rising costs and workers eyeing forecasts which show inflation increasing well into next year.
One transport workers union earlier on Wednesday announced a 24-hour strike next month, while workers of the telecoms group BT, privately owned delivery service Royal Mail and journalists at media company Reach all held separate strikes.
During the 2020 U.S. presidential race and a failed 2016 run at the Democratic Party nomination, Sanders' anti-establishment, pro-healthcare and labour rights message captured the attention of swathes of grassroots activists at home and among leftist movements abroad.