Hi Ivan,
There are a few reasons that I can think of as to why the army would prefer the AH-64E over the AH-1Z (or any other type). Namely:
1. The infrastructure (facilities, training, support, etc) is already in place for the Apache.
2. UK Apaches are already 'marinised' during the manufacturing process, and the AH-64E is also (if you believe Boeing. I know Bell has a different opinion on this). Likewise, they are also C-17-transportable (though granted it takes more work to stow the blades than the AH-1Z - even so, the AH-1Z will only fit into a C-5 without the blades having to come off).
3. Interoperability - while the AH-1Z would make perfect sense for Asian nations such as Australia, that have a large USMC presence, European nations are chiefly looking for compatibility with the US Army - hence the Apache.
4. Better the devil you know - the experience of Afghanistan and Libya (in a maritime setting, no less) proved the value of Apache to the British Army, so why would they want to change for an unknown?
I'm sure those have just scratched the surface, and that others will be along shortly to offer their opinions also.