OK, have taken a look at TERPS and it has an even lower required climb gradient than AlanM says is in use at heathrow. So, taking my earlier question about O'Hare with it's 6nm surface area, from my amateur hack reading it appears to be possible by TERPS for an aircraft to depart O'Hare and then leave the Class B at less than 1500ft AGL.
But then it will enter Class E, which is still controlled, so visibility minima apply. There is also a requirement to have a mode C within 30nm, so any light traffic will be squawking altitude.
The FAA are quite happy with VFR and IFR aircraft being in Class E together with only one talking to a controller, and it would appear to be a possibility at O'Hare. There are lots of Mode C regulations though which mean in practice the light aircraft will be visible on radar or TCAS. In practice most turbines climb out at rather more than 200ft/nm, so it is pretty unlikely that you will be flying around ORD in your spamcan and meet a departing 777 head on.
Heathrow surface area has been designed to keep VFR traffic away from IFR traffic full stop, which is why it is so large (to my amateur mind). In also seems that it might be even more dangerous at LHR, as AlanM says some heavies don't make the required climb, and they could meet a light aircraft without a Mode C transponder on as it's not required.
Difference in attitude by respective regulators, which is what others had said, and probably isn't going to change. Interesting thread which makes one think about it. Seems Mode C is a crucial technical regulatory difference.