The UK domestic rail behind most sterling deposits, moving money bank to broker in minutes. In practice the flow is the same at every FCA firm: you complete identity verification, choose Faster Payments in the cashier, authorise the payment, and the balance appears in your trading account once the broker's payment provider confirms it. Minutes in normal conditions.
The part that varies between brokers is not the payment rail but the treatment afterwards. Some firms credit funds instantly and settle behind the scenes; others hold the deposit until settlement clears, which can add hours. Neither approach is wrong, but if you fund an account to meet a margin call, the difference matters a great deal.
Sterling accounts funded from a UK bank. That is the honest use case, and it is worth being clear about it because funding routes are frequently chosen for convenience and then blamed later for costs that were predictable from the start.