Since Pride and Prejudice is told from the point of view of Elizabeth Bennet, and since her overriding concern is the welfare of Lydia, that looms largest in the text. But the novel makes it clear that Wickham’s main motivation for fleeing Brighton was not to elope with Lydia, but to escape his creditors. Mr Gardiner writes that
in the wretched state of his [Wickham’s] own finances, there was a very powerful motive for secrecy, in addition to his fear of discovery by Lydia’s relations, for it had just transpired that he had left gaming debts behind him to a very considerable amount. Colonel Forster believed that more than a thousand pounds would be necessary to clear his expenses at Brighton. He owed a good deal in town, but his debts of honour were still more formidable.
Jane Austen (1813). Pride and Prejudice, volume 3, chapter 6. Project Gutenberg.
Wickham faced different consequences for the two types of debt that he had incurred. He could be prosecuted and imprisoned for the ordinary commercial debts he “owed in town”, and while this was not the case for his gaming losses, he would be disgraced and forced to resign his commission in the militia for reneging on these “debts of honour”. Wickham confirms this when Darcy finds him in London:
He [Wickham] confessed himself obliged to leave the regiment, on account of some debts of honour, which were very pressing
Austen, volume 3, chapter 10.
Wickham’s plan, insofar as he had any plan beyond surviving from day to day by his wits and charm, was that of “effectually making his fortune by marriage in some other country”.
So his motivation for running away was primarily financial, but since he was running away, then why should he not elope with Lydia? Mr Gardiner writes that Wickham “scrupled not to lay all the ill-consequences of Lydia’s flight on her own folly alone”—that is, he blamed the elopement on Lydia, but it is clear that this was a self-serving lie, since Lydia’s letter to Harriet indicates that Wickham had promised to marry her:
I am going to Gretna Green, and if you cannot guess with who, I shall think you a simpleton, for there is but one man in the world I love, and he is an angel. I should never be happy without him, so think it no harm to be off. You need not send them word at Longbourn of my going, if you do not like it, for it will make the surprise the greater, when I write to them and sign my name Lydia Wickham.
Austen, volume 3, chapter 5.
Gretna Green was the first parish travellers would reach after crossing the border into Scotland on the coach road from London to Edinburgh. At the time of Pride and Prejudice, the Clandestine Marriages Act 1753 meant that people under the age of 21 (Lydia Bennet is 16 in the novel) could not marry in England and Wales without the consent of their parents, but this did not apply in Scotland.
The reader can deduce from all this that Wickham’s promise to Lydia was a lie, and that he had always intended to abandon her the moment he found a more attractive prospect.