What is the best rc helicopter for under £150?

If you do NOT already have the radio, you can forget finding one under your price limit other than toys!!.

You might try the tower hobbies site as they ship worldwide on most items. (www.towerhobbies.com)
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go with the "Hummingbird Elite 3D Pro" its a good heli
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This is a bit of a tall order, as just the electronics can run more than £150 (a good transmitter alone can be this much).

But I don't want to dampen your enthusiasm. What you're looking for is possible, you've just got to know you won't end up with a top of the range set-up (or close to it) for this kind of cash. Here are a few options:

-Venom Night Ranger 3DXL RTF:
http://www.hobbyzone.com/rc_helicopters_venom_night_ranger_3d_xl.htm

- Walkera 60B RTF
http://www.helipal.com/walkera-60b-helicopter-2-4g-metal-edition.html

-A very inexpensive Trex clone:
http://www.xheli.com/exceed-rc-2-4ghz-450-se-rtf.html

Note these sites have a number of other helis that might fit your bill. I've personally bought stuff from all of these guys without issues. But be careful … read the fine print and understand just what you're buying (returns of online gear can be a real pain).
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Years of making helicopter related mistakes…