I have to travel to London later this month. But I just got my passport renewed and so the passport which contains my visa is now canceled. I have all the old passports and my existing fresh one too. So is it a problem if my existing visa is on a canceled passport? What do I need to do?
As long as the visa is valid (not expired and does not have any corners cut or holes punched through it - when the passport was cancelled), you can still use that visa. I had been doing it for a few years prior to gaining British nationality. Simply bring your current passport together with the cancelled passport containing the visa - and you will be fine.
Also note though that possession of a valid visa does not guarantee your entry into the UK. Regardless of your visa situation a border agent may choose to refuse entry to you into the country based on his/her own conclusions about you.