thie first concept you need to get is that an average traveller's visit is only long enough to scratch the surface so to make the best use of that time you need to manage the travel. You shouldn't return home and discover that your main memories of the visit are waiting for trains or staring out of the train window at strings of commuter towns. Get stops in the right order to avoid hours of doubling back. Find out where you can go on a train rather than spending hours choosing locations from photos and then trying to find a rural taxi to pick you up from a station many miles away.
As already said, the National Rail website is the place for sampling schedules by using the current season. This might, possibly, be repalced by the new GBR website before your visit. National Rail have 2 maps showing all routes and stations (over 2500 of them). One is the interactive version in the middle of this page https://www.nationalrail.co.uk/travel-information/maps-of-the-national-rail-network https://www.nationalrail.co.uk/travel-information/maps-of-the-national-rail-network/ when it forst loads it only shows a few staions but click zoom a few times and they all burst into life. The second is the downloadable PDF "All Stations" route map also on that page. It has a major design fault, the font for the station names is far too small so it needs to be zoomed in a lot to make the legible.
If you really intend to do a LOT of travel, check out Britrail Pass. Note the name - it's not UK because Northern Ireland has never been considered to be a part of the British rail network. If you want to go there it's a different set of website, schedules and ticket rules
