Just looking at Bristol to Edinburgh, a quick check on Skyscanner shows 4 departures a day. I don't know what type of aircraft they use but it's easyJet so it'll be between 156 and 235 seats per flight. So not nothing, but less than 1000 seats per day - not actually a huge market for rail to chase.
Rail doesn't just serve a single journey pair though. A single XC train will compete Edinburgh and Newcastle to Birmingham, Bristol and Exeter flights.
For reference though, it is more than 1,000 per day.
Annual totals from Wikipedia's entry on Bristol Airport:
Edinburgh 451,404
Glasgow 292,113
Newcastle 147,923
Inverness 92,198
Aberdeen 14,360
Exeter is much smaller, the only flow big enough to make the top 10 is Edinburgh at 27,405.
Edinburgh to Birmingham is 300,226, Edinburgh to Southampton is 82,130.
Edinburgh to Newquay is 20,867
Aberdeen to Manchester is 62,211
Obviously some of those routes are too long for rail to be competitive even with the best infrastructure, but I reckon that there are around 3,000 journeys per day that could potentially be poached if Crosscountry weren't slow, overpriced and overcrowded. You could fill 15 Voyagers with that.
HS2 Eastern Leg would have knocked an hour off of journey times between Birmingham and Newcastle, and Golborne would have knocked three-quarters of an hour off of Birmingham to Scotland. Each train would have 2.5 times the capacity of a 220. That would have had the potential to knock a substantial dent in domestic flights out of Birmingham.
Integrate Bristol services into Curzon Street and Bristol-Newcastle falls under the magic 4 hr threshold (where rail really starts competing with air) while Bristol-Scotland would be not far above it at around 4hr 30.
I think the railways could take a leaf out of the airlines' book when it comes to pricing.
Oh god no.