The National Archives at Kew, a suburb of London, has many convoy records. Besides the ADM237 series of files dedicated to Convoy Reports, most of which include charts, details on individual convoys can be accessed by different means such as Intelligence files and Convoy Escorts reports. These are all available to the public and can be copied. If you are unable to visit in person, there are people who can research/copy for you. (follow link after my signature for an example)

Having said that, a word of warning: I never recall seeing a statement in the narrative of these files to the effect 'convoy rerouted on basis of ULTRA intelligence'. What you are likely to find is simply 'convoy 30 to starboard' or 'convoy to 270' etc. Forgive me for stating the obvious, but the source of the information was not transmitted to the convoy, and the U-boat Tracking Room daily record book did not survive, so the most reliable sources of the information you require will be the individual accounts of the people who worked in the OIC system. The various volumes of ‘British Intelligence in the Second World War’ provide a handful of specific examples and good clues to the identity of a few more.

Hope this helps,
Bruce