You dont say how old the boys are but why not try some negotiation and give them the three options :-
You change to Premier, they can download what they want when they want but have to accept that their usage in the evening will be throttled. Web browsing speed should be unaffected (so their homework wont take any longer) but they will just have to be patient with their music downloads.
You go on Pro and they get full speed for everything and, providing they are sensible and download their music, videos etc in the free period, there will be no cost to them.
You co on Pro, they ignore your suggestions about scheduling downloads and they then pay for any bandwith used beyond the 20GB.
I've used 22GB in the last 24 days and am on Pro, yet my paid for usage is only around 12GB. A lot of this been down to the full reinstallation of the operating system and all the patches on three machines that I have bought and upgrade as familt christmas presents. It is quite easy to change the settings for microsoft update so it only lets you know that updates are available - you can then choose when to download them.
Your suggestion of 8MB speeds 24/7 for all traffic and 50GB of usage for £20 per month does seem like a big ask with the antiquated infrastructure the UK is using.
My wife was speaking the other day on Facebook to a guy in Egypt (hardly a country one would think of as technologically advanced) and yet he had unlimited 100Mbps broadband!!!! He was absolutely shocked when she said that we had 2Mbps on a good day and that was limited to 20Gigs in what many regard as a first world country.