While the government has "pledged" to fix the current rail ticketing disaster where you can end up paying more than double than you should be for a ticket, it may be a while before this actually happens.
I came across this tool that allows you to search as you normally would for you tickets and it will search if it can be cheaper.
Limited to five searches a day.
Although the advertised way to purchase your split ticket is with Train Line it's cheaper to buy direct usually so you don't encounter booking fees.
Hopefully this can help with the holiday travelling.