my experience/info... at Manager grade, on client site (i.e. away from home office city):
now a max of £240 in London hotels (exceptions need partner approval i think? not sure, never needed it)
first class train travel only if it's over an hour
all flights economy unless it's over 5hrs
25 max food allowance per day (all meals, total daily allowance) but only if if on client site for most of the day, around 19+hrs
(if less hours away, there's bandings e.g. a half day would be £10 food allowance)
£5 incidentals per diem for overnight stays
internal work has very limited allowances, the above doesn't apply at all
in my experience, there's mass confusion about deloitte expenses because a few months ago they changed how you charge them (you now need to put everything on a corporate charge card at Amex, but none of that is in place when you join.. and it took forever to get the card approved/set up/sent out), and there's also been confusion about what rates apply, even when talking to different members of the same Expenses hotline (!).
speaking as someone who's based in a regional office, new to consulting, and was looking for clear guidance - their intranet page on travel & expenses policies is also highly confusing, and literally doesn't make sense in part (broken English, references to points which don't exist in their expense policy).
a bit of a mess tbh!