One thing to be aware of is that fares on at least some routes can climb into the stratosphere if tickets are purchased close to the date of travel. If you have a fixed schedule for moving from city to city (at least for the longer-distance trips), you may save a great deal of money by purchasing your tickets early. The bargain tickets will usually tie you to a specific train on a specific day, however.
You can see this phenomenon in action on the National Rail website. I just looked at one-way fares from London to York. The remaining trains for today (about the next 7-1/2 hours) cost £84.70 to £122. When I looked at tickets for February 20, I found some trains as low as £16, with one super-bargain at £12.80.
One other thought: If you don't have a specific reason for the routing you cite (Scotland-Ireland-Wales-England), you might consider either beginning or ending the trip in Ireland. That would mean one, rather than two, transfers to/from Ireland. Those will require a flight or a ferry and can chew up a lot of time.
You may also find comparatively good airfares into or out of Ireland. Be sure to look for multi-city tickets so you don't have to return to your landing point at the end of your trip.