Having been in telecoms most of my life, and most of that competing with BT I think some of the views on here about hem are a bit extreme.
I use BT for phone line and high speed broadband, well the UK concept of high speed.
Generally I get a good service but I do pay £10 month for UK tech support from people who can speak English (well Geordie English) but are helpful and competent. BTs normal customer support based in India is useless and frustrating.
Perhaps it’s because I know the buttons to push I get a discount or added features on all aspects of my BT service and the only really bad thing is BT Mail which they have botched up transferring from Google.
BT are not the cheapest but not prohibitive , the problem in UK is the scandalous but typical lack of consumer focussed regulation which means companies can advertise dishonestly, spend all their promotion money on new customers and do not give a toss about existing ones unless forcibly reminded. Also unless you are a Virgin customer all ‘last mile’ network belongs to BT anyway and although through Openreach the service for fixing faults is meant to be the same irrespective of ISP you can guess what happens in subtle but important ways in practice