Most of the mags over the years have been bought out of habit, and publishers like to get readers into that, esp. if they get you subscribing; 'inertia selling' is the term for buying into something and then not being *rs*d to get out. If you count the no. of pages you're actually interested by, as against those you flick past, then there's not a lot of value for money in them.
So, my answer, if I really did only have £50 I'd look carefully at the contents and buy 'x', then 'y', then 'z', depending. Anything that's a subscription for the year would wait (I could always borrow from someone for a looksee). And I'd try not to spend the whole £50, as most of the mag contents are on the web in some form or another. Much better spent on the 'book of the year'. If pushed, might've been BB, but would be happy to save the money and wait the couple of years for that to come online as a freebie now..
Mind you, I'm really quite tempted by this new title..
http://thedrunkbirder.wordpress.com/2011/10/13/vague/