I travel up from Kent regularly too & have been dismayed at the prices set by Avanti. Haven't seen the cheap fares on London Northwest to Crewe either.You can go via Crewe on West Midlands trains and back via Crewe on Avanti the week after for £33. It’s £66 for the Norwich weekend. Might be worth looking at LNER and changing at Leeds or Doncaster. You can do that, if booked early enough, for about £55.
What I have seen is LNER doing fares to Leeds or Donny from £17.50 & connect to MCR for £11 return. I'm traveling up this way for the first two home games.
Does mean legging it to Piccadilly for the 17:30 after Norwich but I'm thinking of cabbing it from Ashton New Road to Stalybridge & picking up the service there at 17:45 instead.
Like others have said, don't bother with Trainline, use the national rail site to plan & go direct to providers.
Also, for Leicester away, no direct trains from St Pancras that weekend but £7 or £11 one way to Nuneaton from Euston on London Northwest & £5 return Nun-Leics if you buy separately. You get charged full whack £60 if you try to buy direct.