Heck, it might not be the best magazine in the world, but this seems a pretty decent deal to me, especially if you are after Monster Hunter anyway. They always give out free gifts with the mag too, which is a bonus.
£19.99 for a copy of Monster Hunter Tri and a 6 month sub to Official Nintendo Magazine. Best price for the game alone is £16.99 @ Amazon, so that makes the 6 month magazine sub work out @ £3.
UPDATE: There is also a possible £7 available through Quidco. Follow the Quidco link to myfavouritemagazines, then change the url to http://www.myfavouritemagazines.co.uk/ONMW12/
Quidco tracked correctly at £7 for me, within an hour of placing the order.
Thanks to dnlvry for the feedback on Quidco :)