Obvious answer is that the worst it can get is a full scale an uprising against the British Government and descends into a Syrian style War with Croydon an Aleppo of sorts (sorry Meadow).
More sane and plausible answer would be that the riot becomes extremely hostile and violent as more reactionary and violent groups hijack it (instead of prick looters), with the London and other cities put under Marital Law and a state of emergency declared. Troops are deployed to keep the peace, with curfews enacted and bans on loitering and large groups. The groups would likely be reactionaries and would see sectarian violence, be it on a religious or ethnic level, with many attempting to 'reclaim their streets' as an excuse to attack anyone who is different and fits the stereotypical rioter image. The violence and chaos causes the economy to go south, and Britain reenters the recession hard.