Disclaimer, I have no way of backing up any of this to please take it with a pinch of salt
I have also made the assumption that your meteor is quite big, please correct me if im wrong
Depending how baddly everything is affected the Government will either leave it up to the police to deal with the ensuing chaos, as long as the damage done to buildings isnt overly bad, or if its the coming of the apocalypse then will likely enfoce martial law to try and to keep the peace with force, any military personal affects either as a mutant or with super powers would be quarantined until they are deemed fit for service.
At the same time they would have scientists try to reverse the effects of the people with the photokinetic construct super power, how these power affect people and if you want them to be cured I will leave up to you, it could be something that changes the person physically, the scientists should be able to identify what causes it to happen or its some handwavium and they are affected for life.
The NHS would most likely be crippled and unable to help much in the early days of this event, as random people are being affected by this mutating light meaning their already over worked staff will be forced into worse conditions and with people seeking medical help injurys from the fragmented meteorite or from the mutations, I could easily see the NHS just breaking down completely. If the leaders of the UK are smart they would give alot of protection to the NHS to try and prevent this
we are so screwed.
The Fire department would struggle severely at first as the amount of destruction caused, buildings colapsing/on fire or both, people trapped. If no help is given to the Fire department then its likely that a fair-few people will die due to the fact that they cant help everyone. If they do recieve help, from any source, then alot less people will die.
After the first few weeks/months and help from either military and/or volunteers the chaos would start to die down a bit, as long as its done correctly. If they tried all this and did it sloppy and ineffectivly then its likely that unrest would rise with the population riots would become more and more frequent and the Government would become more heavy handed leading to a civil war, but lets assume this didnt happen.
Basic overview of what might happen
The UK population would go into mass panic as they were expecting to either be saved or just have London obliterated, people would likely believe that they would be safe as long as they werent close to London.
When the Army gets involved any raging mutant would likely be shot on site or possible capture of a few for scientists to study.
NHS, Fire department and Police would would either be supported or support the army in restoring peace.
After the initial panic by people they will look to the Government for answer as to what is happening and what they are going to do/doing about the issue at hand. The Government would say they are doing what they can and everyone needs to be patient and just do what they are told to do.
At this point it would be body count and total damage done and plans to see what should be done first,
if they bailout the banks again then please set the country on fire, food and water should be first, after that they should have tempary shealter for everyone who needs it, reconnecting families would be important (there would be deceased but they would be dealt with differently)
The first year after this event would be hectic but as long as nothing major impedes the repairs to the country (IE mass death of everyone) then it would slowly start to come back to live again.
On a side note, people who lost loved ones and friends and family would likely blame people with the super powers and there would be quite a bit of discrimination agaisnt them.
This would only happen after several hundred meetings within the Government, thousands of hours spent debating the right colour of the logo needed to represent this disaster and millions of pounds towards the creation of a flag