I have been using Nevex Printing Centre at Kings Cross. They are open every day till late and I usually get my prints the same day. They have a book with samples of the paper and card and I like to check that before I print my documents. If you know what you are after, you can place the order over the phone or email. I don't know if they are the best but they are perfect for my needs as a student especially for last moment printing.
visit https://www.nevex.co.uk/ for more info

I ordered 3 A0 posters with delivery. They were delivered same day to me and I was very happy with the quality. I would recommend them if you are looking for cheap and quick print service.