All this is just opinion and feel free to prove me wrong g.
I don't think there's good and bad companies as far as speed is concerned, I believe virgin use their cable and their service is different to others, but all the other companies use the same lines, whether fibre or not. If they don't throttle your connection back (which they shouldn't anymore) then your speed is determined ed by your distance from the telephone exchange and/or distance you are from the box your street is from where the fibre optic cable actually end (cause realistically there's very few households that have fibre cable to the door)
I was with sky and had great speed (16.5 - 17mb on a 17mb line) I am now with talktalk a d get unlimited 38mb fibre for £22.50 a month(prices include rental by law nowadays) (average 36.5-37.5mb). And those speeds are over wifi.
I wouldn't personally entertain virgin for my own reasons which is good as they don't have their cables on my road. It should be said though, I'm only about 400m from our telephone exchange, not many people realistically get the speeds I achieve.
The step from 17mb to 38mb line isn't really noticeable until you download big files or try to use games console and TV and 3+ devices all running at same time - these companies will have you believe you need their fastest line just like bike manufacturers sell us newest components etc.. The biggest different centre for say games consoles is to use a wired connection and not wifi, as this greatly reduces "ping".
So, see what your neighbours average, or even check online as there are sites that will have average speeds etc. I would say to have a minimum of 8mb average and you'll be able to watch TV and do browse g on Internet on a few devices at once - using YouTube or other videos may slow it but 4mb is enough for a TV if your not watching 4k resolution etc..
And I have known people who have had free upgrades from virgin, I think you shod dispute the matter and tell them your not willing to pay it, you signed to 50mb and it was their choice to upgrade you... Others have got it for free and got the extra costs refunded (though not all of them).