If you miss your train on a UK Advance Single ticket in 2026, your ticket is technically void and you are required to buy a new one at the "walk-up" (anytime) rate, which can be significantly more expensive. However, there are two major exceptions. First, if you miss your connection because a previous train was delayed, the rail operator is legally required to accept your ticket on the next available service, even if it's with a different company. You should get your ticket "endorsed" (stamped) by a guard or station staff to prove the delay. Second, under the "Booking with Confidence" schemes, some operators may allow you to change your ticket up to 24 hours before travel for a small fee. If you simply arrived late at the station due to your own error, your best move is to talk to a member of the station staff immediately; while they aren't obligated to help, they have the discretion to let you on the next train if you are polite and the delay was minimal, saving you from a costly new ticket.