Like others, I really cannot see why OR supplied services are not done the same. It doesn't seem to involve more kit in the home i.e. having the same up and down speeds.
TL;DR: 2) the technology is asymmetric, Openreach don't want to saturate the PON so a ratio of 2:1 download:upload is a hard limit. 1) Openreach want to make more money so the ratio is more like 8:1 at the high end.
There are two reasons that make sense.
1) Protection of leased line revenue. 110Mbit upload is plenty for any home use, even with multiple residents on different video calls, streaming (out) and so on. It is also fine for small to medium offices, if there are no special needs such as engineering firms with large drawings, media production etc. 110 up is likely comfortable for 20 users at least. If there is a business need for more than 100Mbit then there is money to spend on a leased line. Openreach supply a lot of those and don't want to lose that income.
In some city centre business areas (Birmingham, Manchester, Edinburgh, probably more) FTTC has not been widely deployed, some say as a ploy to sell leased lines. A typical small business (say 10 in the office) might take a 50/50 or 100/100 leased line. When FTTP comes along Openreach don't want them moving to a cheap 80/80 service, which would be fine for them. Keeping it asymmetric at 80/20 means they have to pay for 550/75 or 1000/115 to get the upload they need.
Alternative networks don't have that base so there is no harm in offering symmetric speeds especially in residential areas. ISP traffic is heavily inbound and their upstream transit and peering links will all be symmetric, so moving the traffic really does cost them nothing, and it's an easy way to differentiate their service from Openreach. On top of that, apart from the odd work-from-home engineer, graphic designer or photographer, nobody will make use of the upload. Making money by selling something you have sitting on the shelf (upstream capacity) and that nobody will really use is excellent business!
2) OR FTTP runs on GPON which shares ~2.5Gbit down and ~1.2Gbit up between up to 32 connections. I imagine they don't want to deal with situations where the PON gets saturated so they've set contention at a level they know won't cause problems for a while. By that argument they could, however, set upload bandwidth to 50% of download.