When not doing my own research, I'll use the following:
https://raileasy.trainsplit.com/Very comprehensive, with plenty of advanced options to really drill down on the best split for your itinerary. It also clearly separates Advance and walk-up fares. Remember, Advance Purchase tickets usually come with a seat reservation (definitely with CrossCountry should you choose to go via Birmingham), so split Advances can mean musical chairs. If you buy split walk ups from a station you can try booking a through reservation at the same time, but there's no guarantee you'll get a booked seat throughout. Particularly with CrossCountry who allocate very few seats for reservation away from their own sales channels.
You don't have to buy from 'trainsplit.' Although they factor in a percentage of the split savings as their fee, they also show a breakdown of all the tickets needed for a particular split before you get to the payment screen. You can take this information and buy the tickets yourself, avoiding the fee, or you can pay 'trainsplit' for doing the legwork. Your call.
What 'trainsplit' won't find for you though are the anomalies in routeing, fares and easements, and it won't tell you how to take advantage of the National Rail Conditions of Travel for potential savings. Using the Routeing Guide, Easements and NRCoT are, I'd suggest, not for the 'novice' split ticketer, but if you are interested in one particular loophole for your journey then I'll happily explain it here.
I could take a look at some options for you, with a little more afro. Dates? Number travelling? Direct with (mostly) CrossCountry, or via London? Ballpark departure/arrival times? Railcards?