The 90%+ number is "fibre internet" not "Fibre to the Home". Most of it is "Fibre to the cabinet" (Cable, VDSL and G.Fast connections are the most common FTTC in the UK). Just saying thats where the number comes from and why said it included cable,fttc and "true fibre" in the numbers.

I also depend on the internet for my work too (And for my home entertainment - Games, NF, Amazon Prime, YT, etc etc etc). The reason I used 4G over fixed line is because at the time I was living in a rental place caring for my parents and my shithole of a landlord refused to allow me to have my own fixed line installed. It was eaiser to just use 4G then deal with the battle as I was only going to be living there for 12 months max.

Total complete downtime on 4G over the 12 months I depended on it for me was 2 days (1 of which was a nationwide outage - I forget the reason for the other day. I did have someone drive into the local mast - that was fixed within a week iirc but during that time I still had a connection, just a bit slower then normal). I've had similar downtime (days per year) on my fixed line providers over the past 20 years I've been paying for my own connection. (I had someone drive into my local phone cabinet back in 2016 which took it out completely https://i.imgur.com/CBiLSV7.jpg that took Openreach a week to fix)

The reason I stopped using 4G for my day to day internet is because I moved back and I could get a faster connection via fixed line.

The reason I say to try 4G is because a decent 2nd hand 4G router is going to cost less then £25 on Ebay and Three's pay as you go mobile broadband costs at most £35 for a month (No comittment pay as you go - but if your inital tests work out well for you then you can commit to a longer contract and lower the monthly bill) so even if it sucks for you you are only out £60 but if you wanted to try for even less you can just buy their 4GB payg sim for £10 and do a speed test on your own phone (though I don't think Three allow tethering on that plan - but doing a speed test on your phone will give you a rough idea of the speed you should be getting).

You can use your own phone as a modem however you are gonna be limited by the phones USB/WiFi speed and its a bit more of a pain in the ass to set up somehting like OpenWRT to share your phones connection with the rest of your house and when you leave the house with your phone your house loses internet connectivity - but using your phone would be a decent speed test for you before dropping the coin on a 4G Modem/router.

But I will say try before you commit to a 12 month contract because even though I was getting 100Mbps at the rental place on 4G when I go to my friends place I only get 70~ but as its wireless you can try the modem is differnt places around your home to see where you get the best speed. In the Rental place I was on the first floor and I placed the router on the window sil closest to the mast (There are websites that will list the towers around you.)

EDIT: The reason I say Three is because they were the best network for me at the time, you might have better luck with one of the other networks. But that is why you test with their no contract options first before commiting to a cheaper contract.

EDIT2: For me, my local mast was not over subscribed so I had no issues even during peak times but that is location dependant - mobile providers have been upgrading their backhaul in recent years (mainly in prep for 5G) so honestly it is worth reconsidering. I know I would be if I was stuck with a ADSL line (That or be planning to move. Connectivity is the new "good transport links" when house shopping, The house might be beautiful and a nice location and at an awesome price, but if the net is shit I aint moving in.)