Cheap train tickets. Help pleaseWatch
Note that with advance bookings, you are normally required to travel on a specific train. I believe you can book up to 12 weeks in advance, and the cheapest fares are first-come-first-served, and limited numbers are available.
Try using the National Rail Enquiries web site (https://www.nationalrail.co.uk/) and see what pricing is like and how it varies.
Also, note that pricing doesn't necessarily have to make sense -- sometimes two singles can be cheaper than a return; sometimes "walk-up" pricing can be cheaper than advance booking. Also, as suggested, look up "split ticketing".