I see 2 conflicting regulations:
1. UK CAP 413, which specifies "Ready for Departure" by the pilot; and
2. ICAO Doc 9432 - Manual of Radiotelophony; ICAO Doc 4444 PANS-Air Traffic Management, which both specify "Ready" by the pilot.
In no situation does the pilot use the word "take-off" UNLESS when acknowledging the take-off clearance issued by the tower.
While operating in the UK, CAP 413 takes precedence; operating in the rest of the world, ICAO rules OR the local state rules take precedence.
BTW, a similar conflict exists between the UK climb (SID) and descent (STAR) altitude clearances and ICAO/rest of the world, and must be treated with extreme caution to avoid altitude busts. Refer UK CAA 'FODCOM 16-2009'.