I'm eagerly awaiting JT here to answer this, but ITMT, check JAR which will define the 4 (poss 5) stages of 'take-off' - normally up to 1500'AAL.
Consider now the fact that e/out perf gives you a ?35'? (not sure, but not much!!) clearance on any obstacle in the flight path and ask yourself how much loss of height you will get with Eng a/ice on and is it crucial?
In the case you quote you will probably be using a/ice from around 1000' when you decide to put it on until clear of cloud, but at least until 1500'AGL unless the cloud is very thin. If I assume your example is LHR 27R, then I would suggest (and wait to be pilloried for it!) that it is not that crucial (unless the standard is flying on Windsor Castle
)? Why not ask your Chief trainer?