Sounds like you saved the life of a special bird. If it was that cold outside, he couldn't have been out more than a day or so, or flown very far. I wouldn't think more than 5 miles (10 with a good tailwind). However, in London that includes an enormous number of households.
He could have escaped his owner, which is the most likely scenario, or he could have been stolen and perhaps released by the thief. Whatever the reason, you could check with local vets, the SPCA, animal shelters (the owners will be checking there), or look for posters in pet stores. You might also get him to a vet with a chip reader to see if he is micro-chipped.
I recommend at least 50% of the bird's diet be a pelleted diet for all large parrots. Its the most balanced diet and one of the simplest. Get some small bags of a few to see what is most palatable for your newfound friend. You can supplement it with occasional fruits and veggies. Mine eat about anything I will eat. I also treat them with occasional whole grain breads, oatmeal, rice, pasta, well cooked chicken bones, cheese, yogurt, etc. If it's part of a healthy diet for you, it is probably alright for them. No chocolate or high sugar treats (candy, rich desserts, etc).
I give my birds fresh pellets in the morning and let them have access to them all day. At night when I get home from work, I use the other foods as 'social' foods. They are treats/rewards for socializing with me and my wife. If someone gets nippy or out of line, they go back in their cage and get no more treats. They figure this out pretty quickly.
I would recommend you trim the wings on this bird. That's how the first owner lost him. The bird might not survive a second experience with the great outdoors. It will also give you more control over him and make him a little more dependent on you as you two form your new relationship.
Find a great (one you really like) avian vet. They are expensive but WELL worth it in the long run. Birds hide any symptoms of illness right up until the last minute. By the time a bird looks sick, it is VERY sick. You always need to stay ahead of the game when it comes to a bird's health. Their metabolism is so fast that when things go wrong, ...they go wrong in a hurry.
Get some books, pick up some bird magazines and read as much as possible. One of my favorite websites, filled with friendly people who will help you is Parrottimes.com
. Jodi, the lady who runs it used to be the bird mod here. Super lady.
Just like any type of pet, there are numerous ways to keep them. You will no doubt find people who are not 'pellet' people. There is also school of thought that hates clipping wings. I'm giving you my advice from my point of view based on my experiences. Your mileage may vary....
Feel free to PM me here or on Parrottimes if you have any questions.
(and BTW, welcome to Paw Talk!)