In the original source comic, it would appear that the reason V was so keen to accept Evey's offer of help was simply that it was convenient to his planning. Presumably he'd interecepted one of the Bishop's semi-regular communications with the pimp who supplied his underage girls and knew that this presented an opportunity to attack him at a moment of weakness; both in terms of his personal security and in terms of his moral frailty.
Notably, in the original version of the script, the line remains much unchanged
EVEY: I want to help you, V. I want to do something. Can we make a
V: Yes. I think we can make a deal if you like. I think I know a way
you could help me very soon indeed.
If I had to guess, the "unforeseen circumstances" line reflects a slight change between the comics and the film. At this point in the film Finch has already discovered the connection between V and Larkhill (by looking at Prothero's record), something that doesn't occur in the books until far later (when he discovers the Doctor's diary).