Speyer was a supporter of the musical arts and a friend of several leading composers, including Edward Elgar, Richard Strauss and Claude Debussy.

In Chicago, Yerkes had led the development of the city's urban transport system, and he went to London to capitalise on the emerging opportunities for new deep-level underground "tube" railways there.

He and Speyer headed a consortium of international investors involved in the construction of three of London's underground railways and the electrification of a fourth.

[note 2] With Yerkes as chairman, the Underground Electric Railways Company of London (UERL) was established in 1902 with a capitalisation of £5 million, the majority of shares sold to overseas investors.

After bailing out the company,[note 6] Speyer, with Managing Director Albert Stanley, struggled for a number of years to restore its finances.

He owned a pair of neighbouring houses at 44 and 46 Grosvenor Street, Mayfair, that he had rebuilt as a single residence at the cost of £150,000 (equivalent to £19.3 million in 2023).

[8][17] The rebuilding work was carried out by Detmar Blow and Fernand Billery in 1910 and 1911; the architects gave the house a "Beaux-Arts" style portland stone façade and lavish interiors including 11 bedrooms and a large music room.

[1][note 10] Like his cousin Edward Speyer, Edgar was a music lover and patron of the arts, frequently holding concerts in his home.

[1][26][27] Speyer increased rehearsal time for the Queen's Hall Orchestra and was involved in the challenge to the deputy system then operating, stopping musicians from sending under-prepared substitutes to perform in their places.

[8][36] In December 1904, having read of the loss in a newspaper article, Speyer donated £5,700 to replace all of the funds lost by investors in the failure of a penny savings bank at Needham Market, Suffolk.

[37] From 1909, Speyer was honorary treasurer of the fund raised to finance Robert Falcon Scott's 1910 British Antarctic Expedition to which he donated £1,000 of the £40,000 that was required.

[38] Speyer was prepared to take personal responsibility for a share of the liabilities of the expedition, although the money raised from public donations was sufficient.

Following the British declaration of war with Germany on 4 August 1914, Speyer resigned as a partner of the Frankfurt branch of the bank.

After a Royal Proclamation on 11 September 1914[46] requiring British subjects to have no links with companies doing business with Germany, Speyer resigned as a partner of the American bank.

Speyer was asked to resign from the board of the Poplar Hospital due to threats of substantial reductions in donations if he remained.

[48] Speyer ignored a call to write one of the "loyalty letters" that Sir Arthur Pinero proposed be provided by prominent naturalised citizens of German origin.

[49][50] Instead, on 17 May 1915, Speyer wrote to Asquith, then Prime Minister, asking him to accept his resignation as a Privy Counsellor and to revoke his baronetcy, stating: Nothing is harder to bear than a sense of injustice that finds no vent in expression.

But I can keep silence no longer, for these charges and suggestion have now been repeated by public men who have not scrupled to use their position to inflame the overstrained feelings of the people.

But I consider it due to my honour as a loyal British subject and my personal dignity as a man to retire all my public positions.

[51][52] It is doubtful whether it was possible for Speyer to resign from the Privy Council or as a baronet, there being no normal mechanism to do so,[53] but the Prime Minister's response was supportive: "I have known you long, and well enough to estimate at their true value these baseless and malignant imputations upon your loyalty to the British Crown.

The King is not prepared to take any step such as you suggest in regard to the marks of distinction which you have received in recognition of public services and philanthropic munificence.

[57] Speyer again offered the Prime Minister, then David Lloyd George, his resignation from the council, but received no response.

Speyer stated that the issues involved were of a trivial nature and were similar to those encountered by other British banks which had traded without censure.

[74] Leanne Langley suggests that the character of Appleton, a villainous stockbroker, in John Buchan's The Thirty-Nine Steps may have been based on Speyer.

[75] After the American Women's club moved out, his London home served as the Japanese Embassy for some years and is now the offices of stockbrokers Killik & Co.[18][76] It is a Grade II* listed building.

[79][80] Vivien came to Britain as a member of the United States Women's Army Auxiliary Corps and died in Norwalk, Connecticut, in 2001.