The Landsend Transit Corridor dimensions are changing. It will start at St Ives and go up to 3000. The Latest AIRAC has the exact details
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The Land's End Transit corridor is a funny thing. I was talking to one of our group recently who said "you have to cross at 2000'" - thinking it was a mandatory route. Bollocks to that I said to him, I'll be crossing at FL60-65 talking to Culdrose for a traffic service if possible. Why not? Go as high as possible, there is no airspace above and you're less likely to run into any helicopters or other flights.