You could use DA from the declared conditions charts - CAO 20.7.0, and look at 20.7.4 for what you're supposed to be allowing for.
Realistically, I'd try to get some idea of what the real temperature and atmospheric pressure's going to be like on the day from the ARFOR and any local met info you can get, work out predicted and worst case scenarios from the charts to see if your proposed operation is feasible in the first place.
Depending what you're flying and what load you're carrying, takeoff's likely to be the most critical strip-length wise, so you could check the temperature and read off the QNH by setting airfield elevation on the altimeter when you're there and confirm the figures you've previously worked out, or crunch it again on the spot if it's tight.
That's what I'd do anyway, others may have suggestions.