Using the AppShopper app you can easily weed out all of the apps that drop to "free" on the iTunes app store.
Filter out the what's dropped to free by setting it up like so (screenshot in first comment below):
Type: Price Drops
Device: Choose if you want a specific target (iPhone, iPad, Universal)
Category: Set it to search for specific categories if you like.
This app has gotten me plenty of freebies when it comes to apps dropping to the low, low price of 0.
If you go one step further and register, you can set up alerts when apps you want drop in price or even go free with push notifications.
I've easily gotten hundreds of pounds worth of apps out of this single application so it's worth a try for those that didn't know about it and want a bargain.
Oh yes, remember to set your region to UK in the settings too to get GBP!