Hi everyone - Thought I'd add to this slighly old thread as I have some useful insight.
I live in the Bristol area and work in London (city) 3 days a week, sometimes 4. I do a mix of commuting up/down the same day, and stay in hotels sometimes. The commute is 2 hours door-to-desk (15-20 mins drive to Parkway, 1h15 on the train, 10 mins to Liverpool Street on the Elizabeth Line plus a bit of walking/waiting).
So on the up/down days I'm spending 4 hours commuting, but it's quite broken up and the 1h15m on the train flys by as I can work, watch movies etc. The trains are quite comfy and I always get a seat, usually a table seat. GWR is generally reliable but has issues like any train company, so I've lost a few evenings stuck in Reading or similar.
I typically leave the house at 7:30am and home by 7:30pm, which is fine and not much different to when I lived in the London commuter belt. This will seem like a very long day if you're used to living near work, but I manage it no problem 3 days a week.
When I do hotels I usually stay in London mon-weds so only 4 hours commuting all week, which is nice, but do miss being at home so I'm mainly doing the up/down commute these days.
It's expensive, probably double the cost of commuting from the London belt, but cost of living is lower out west which compensates.
It's not for everyone but we've made it work with no regrets after 2 years. We much prefer living in the west country to the boring commuter town we left behind. Kids are thriving in great schools with loads of friends, we live in a rural village location with an amazing vibrant city 5 minutes away, and can be at the coast or up on the mendips in 20 minutes.