Whether it's easier or not depends on how much you know. That sounds stupid, but it's sadly the case. If you do the H/W at Gatwick, they deliberately fail to mention the most critical aspect of the test; that the hue shade) of the colours will change during the test. Greens, Reds and Whites can be light or dark... By not telling you this it becomes more of a mental test than a colour test.
Having done both I would say they're fairly similar, but that key piece of knowledge makes all the difference. I failed the H/W at Gatwick, almost certainly by not knowing about the hue changes, yet when I took the Spectrolux it was explained fully in advance. I'd take a bet on passing the H/W now I know how it works. In fact, I'm at the Belgrano in a couple of weeks; since I seem to have made something of an impression with the medical crew I might just see if they'll let me have a go. For research purposes of course...
There used to be 2 Spec lanterns available (that I know of). 1 was in Dubendorf, the other at Spiez; both in Switzerland. I'm told the former is no longer available (shame really - it's a great street to dance down on the way to the local bar after passing
). Both are/were the responsibility of Swiss Aeromedical. There are several forum members who have been to Spiez recently, some with very positive results (ie. passed). It's not nearly as easy to get to as Dubendorf unfortunately, but still accessible thanks to Switzerland having a transport system 1000x better than the UK.
I'm sure some of our similarly affected colleagues will be along shortly with further Spiez details. Best of luck if you decide to go there.