Fee free changes that I'm aware of:
CrossCountry (
https://www.crosscountrytrains.co.uk/tickets-savings/offers-discounts/change-advance-train-ticket). States can be changed for free until 15 mins before scheduled departure. Charges refund fees to change or refund non-advance tickets.
TransPennine Express (
https://www.tpexpress.co.uk/help/change-my-ticket). States can change
both advance and flexible tickets (as long as origin and destination remain the same) fee free. Deadline for free advance changes given as 6pm the day before. Refunds of flexible tickets without rebooking looks to attract refund fee.
Transport for Wales (
https://tfw.wales/help-and-contact/rail/refunds) Changes to advance tickets only fee free until 6pm the day before. Fees apply to refund flexible tickets.
Ticketyboo (
https://myticketyboo.com/faqs/refunds-and-compensation/). No charges for refunds or changes to either advance or flexible tickets. Note: no telephone number, email contact only.
RailSmartr (
https://railsmartr.co.uk/faq_list/refunds-amendments/) No charges for refunds or changes to either advance or flexible tickets. Also offers split ticketing without additional fee (but the split ticket search may not be as in depth as the RailEasy engine used by services like this forum). Can change advance tickets right up to scheduled departure.