Sky are doing SOGEA so it’s VoIP with g.fast, they supply a Sky SR203, it’s OK but nothing special, you plug your phone into the back and yes, if power fails you loose landline facilities, given everyone has a mobile it’s hardly a massive issue - you have the same result in a power cut with cordless phones.
As to not trying something because it may not work, that’s just silly. OR and ISP’s have no interest in selling products that result in abnormally high faults/customer complaints as it inevitably costs them money and customer churn/reputation damage. I have g.fast, I had initial issues (BT’s fault), I kept pushing BT/OR, got not one but two deadlock letters till BT eventually did a massive (and expensive) u-turn, service was fine for over 18 months, switched to Sky, it’s been fine for as long as I’ve been with them. If you want to run your own router, it’s DHCP option 60/61 support that’s needed, it’s not MER and ignore anyone who mentions wireshark, they are talking about a process that hasn’t been required in ages.
To use your own router, you need a g.fast modem. They don’t fetch quite what they used to 18 months ago, but they still usually do over £100 on eBay.