DNS level is too easy to circumvent - you just set your server to 1.1.1.1 and 8.8.8.8, and it's enough just one of them refuses to kowtow to Ofcom. More likely it'd de on routing level. Which has obvious problems unless they build full-blown DPI firewall and implement VPN blocking (they may consult China for that) but to cut off the most lazy users it'd be effective.
