Heathrow Director,
I'm going to guess here and state that the decision to deviate, where and when, is probably the result of past experiences, coupled with company SOP. In the USAF we have pretty strict criteria concerning T-storm avoidance, above FL230 20 miles, below 10 miles, At my airline job its whatever your judgement decides. Also If a pilot devites 5 miles and gets clobbered and another finds 5 miles works fine you'll have two different requests. Having been caught in a Heavy T-storm in a T-37 (no Wx radar) I can tell you I err towards the more conservative..