By the death of Elizabeth in 1603, James was the only realistic candidate for the throne. This was not by dynastic chance, but because James had spent the previous two decades ensuring that he would be the next King of England. In 1586 Elizabeth and James signed the Treaty of Berwick, which established an alliance between the two kingdoms, and Elizabeth pledged to not interfere with James’ claim to the English throne. Though the two monarchs had multiple disagreements over the intervening years, especially the execution of James’ mother Mary, Elizabeth kept her commitments made in the treaty. James, for his part, began and maintained a consistent correspondence with Robert Cecil throughout the 1590s in order to ingratiate himself with Cecil and other elites in London. James’ ploy was successful, and helped ensure that he was the only realistic candidate when Elizabeth died.

The other claimants to the throne never stood much of a chance, but that did not prevent James from either expressing concern or taking diplomatic action to prevent the other claimants from gaining a toehold in England. In 1594, a rather scandalous pamphlet was published in Antwerp entitled “A Conference about the next Succession to the Crowne of Ingland” (STC 19398). The author was likely exiled English Jesuit Robert Persons, and in the work Persons argues for the primacy of the claim of Isabella, daughter of Philip II of Spain. The work was controversial, and many English Catholics published pamphlets and open letters against the text. These writers wanted to reduce the tension between Elizabeth and Spain, and saw the book as an attempted provocation; they also argued that the work would only serve to feed anti-Spanish rhetoric and persecutions. The work came to naught, and its author Persons tried to back pedal from his radical position established in the work by 1600.

Philip III did try to encourage Isabella’s candidacy for the English throne in the years leading up to 1603. His diplomats tried to feel out the English elite’s opinions on a Spanish succession, what they thought of James, and if there were any other legitimate candidates. Philip and his informants concluded that there were no other candidates with as strong of a position as Isabella. Despite his diplomatic intrigue and attempted invasion of Ireland in 1601, Philip was forced to resign his attempts of putting Isabella on the throne (Ireland was in rebellion in 1601 when Philip sent 4,000 men to meet up with the rebels. The invasion eventually failed due to poor planning, and insufficient men or resources).

The other major candidates, according to Susan Doran, were Lady Arabella Stuart (cousin to James – her father Charles Stuart was James’ paternal uncle) and the children of Lady Katherine Grey (descendants of Mary, daughter of Henry VII. Genealogy: Henry VII -> Mary -> Frances Grey (ne’ Brandon) -> Lady Katherine Grey (d. 1568)). Neither candidate had a strong claim, but that did not assuage James’ concerns. Lady Arabella never tried to claim the throne for herself, and she was initially on good terms with James. However, she entered into a secret marriage without James approval in 1610 and was removed from court. The descendants of Katherine Grey had claim to the throne through the provision of inheritance in Henry VIII’s and Edward VI’s wills. Katherine and her sister Jane were heir through Henry VIII’s sisters, and Jane had been executed for her attempt to seize the throne from Mary Tudor in 1553. James was hostile to the Grey family (also known as the house of Hertford), and any favor Elizabeth showed them put a strain on Anglo-Scottish relations.

To sum up – there were about three people who had claims to the English crown at the death of Elizabeth. However, Isabella of Spain would only have been possible to make Queen if it was done through military force, as the idea for a Catholic monarch of England was repugnant to the populace and elites. The other relatives of James, Arabella Stuart and Katherine Grey’s children, never had much of a claim to pursue, and Elizabeth did her part to hinder their claims. James was the only realistic candidate by the time Elizabeth died in 1603: he had been in consistent contact with Elizabeth’s ministers, he attempted to keep peace at the border between the two kingdoms, to support Elizabeth in her dealings with the Tyrone rebellion in Ireland, and (thought she never said it explicitly) James had Elizbeth’s support for his claim.

Sources: Loomie, “Philip III and the Stuart Succession in England”; essays in Doran & Richardson, eds. Tudor England and Its Neighbours; ODNB Online entries for Lady Arabella Stuart, Lady Jane Grey, and Lady Katherine Grey; Holmes, “Authorship and Early Reception of a Conference about the Next Succession.”