I find the size of the difference depends on the style you're brewing. In a moderate to highly hopped American Pale Ale, the difference is noticeable, but not huge. In a low gravity, lightly hopped, Ordinary Bitter (where the malt plays more of a leading role) it's a very noticeable difference.
I wouldn't do it to save a few bucks, but if that's your angle, give it a shot - it might work out fine for you. If it's merely an availability issue, Golden Promise is a good substitute.