We have used I Can Hire - their vans of differing sizes are based all over North London (maybe even London) and you can rent for as little as an hour. We rented a van for 3 days to move but also for an hour on Sunday just gone to collect a fridge.
I used Bill the Van Man (recommended by Finsbury Park Ranger) earlier this week. Really lovely guy, efficient, £40 for a one hour light removals job for me (lots of boxes and suitcases) in the evening. He'll definitely help with the carrying, not sure whether he does installation but you could ask. He does short notice jobs: 07789 777302