I've been addling my brain on why Wickham took Lydia to London. It's likely Wicham deserted his Regiment due to debts accrued at Brighton (and he was probably tired of life in the militia.) And he's the type of man who'd use women before flinging them away after tiring of them. I'm sure Lydia found him a charmer and he found her physically attractive but he doesn't appear to have much affection for her. She has little dowry to extract. There were plenty of Lydias in London for Wickham. While I can see why for Lydia it would be an adventure and 'a joke.' I can't see why Wickham bothered to take Lydia to London with him.