Bet check that again mate it isn't illegal you just need to be sight of ground.
Thanks, although you are possibly missing the point of my comment. Are you also forgetting the requirement for 1500 metres visibility for VFR?
The only reason I fly IFR under the London TMA (or any other) is when I'm obliged to because VFR isn't possible due weather i.e. there's no other option to get from A to B due to low cloud. The airspace is from 2,500 feet up and the MSA is 2400 feet for much of the region we operate over. If the cloud is low you have no other way of operating legally.
I do operate iaw with the 1,000 feet above the tallest obstacle within 5nm of track, rather than taking the sector altitude, which in some areas gives a bit of leeway and if it's there I take advantage of the extra altitude buffer, unless other traffic is an issue.
Obviously, the 1,000 foot (low flying) rule must also be taken into account. Either way, sometimes it's necessary to tuck right up under the airspace. The London controllers are quite used to this because that is the way the airspace was designed.