If you are using Modules ... Order Total ... Shipping ot_shipping and setting Free Shipping on Orders >= 50.00 ...

Then you would clone Flat Rate flat and make:
flatuk
flatworld

And Zone those for the UK and the Rest of the world ...

Or, just use Zone Rate and set the first for:
Zone 1
GB
49.99:2.95

Zone 2
00
49.99:5.95

NOTE: 00 means The Rest of the World not otherwise defined ...

NOTE: you don't have to use 49.99 as *any value* in this case from 49.99 would work as the shipping module will be turned off or disabled when the order total hits 50.00 by the Shipping ot_shipping setting ...

NOTE: if you do not see a reference to 00 in Zone Rate when editing, you need to uninstall and reinstall Zone Rate as this is new so you don't have to define a bazillion countries ...