I've always argued that if Thorn fits a component it is bound to be a good-value, long-lived, no bulldust item worth the money you pay for it. It's a very attractive facet of the Thorn management that they don't saddle their clientele with the costs of passing enthusiasms and ephemera that burden the customers of lesser bike-makers.
But if you want the best, it is your money.
Going by reputation among anglophone cyclists, especially the Americans, Chris King is the very best. I have no experience. In fact, in 30 years of cycling, I've never met, either in real life or on the net, anyone who spent his own money on a Chris King headset, though I know some who have Royce and Phil bottom brackets bought with their own money, which are in the same class of repute.
I use Cane Creek headsets, notably the S8 which many engineers think was when the threadless headset reached its pinnacle.
Why? The minute you step outside the anglophone/American bubble, and start talking to engineers rather than marketers, those old Dia-Compe/Tange threadless headsets get a lot of love. Dia-Compe USA was the firm which licensed John Rader's groundbreaking design and developed it into a world beater. In time, Dia-Compe USA would split off from the Japanese parent and become Cane Creek, after the street in which their office stood. (1)
I like buying the products of the inventors from the inventors. So do a lot of engineers and especially German engineers.
The last time I looked, years ago, a Cane Creek S8 cost £63. For
large OEMs, who could probably buy it for £25 or less, that was still too much because it would add £100 to the price of the bike, not something to be lightly ventured in competitive markets. For a cottage industry bike makers such as I patronize, the headset price would be pretty near £50 and would add £2-300 to the delivery price: totally impossible.
However, Humpert, a German distributor with serious engineering chops among their staff, offers a copy of the Cane Creek S8, built by Cane Creek off the original blueprints, without the Cane Creek overheads. Cane Creek's name is also on my Humpert Aheadset, though in smaller type than Humpert's. But Humpert's price is significantly lower than Cane Creek's. So you find the S8 on a lot of rather pricey top-level bikes in Germany and The Netherlands.
Sorry about the focus on the photo. The big white text disappearing round the front is the Humpert model name, X-act, and the smaller text towards the rear of the headset topring is Cane Creek's more modest statement of responsibility.
My own pet German baukast hires an engineering firm to test every component considered for their bikes to destruction. They're convinced the Humpert S8 is the right stuff, and since they have skin in the game (a ten-year unlimited mileage guarantee), their opinion counts with me, and besides I already had Cane Creek headsets on several of my bikes.
SJS offers a Cane Creek 110, which as Dan has already pointed out comes with a lifetime guarantee for £109, which is probably equivalent to an S8 for £63 if you take inflation into account.
I were you, I'd look into Humpert's rebranded Cane Creeks and save the difference for another component that merits improving.
(1) Here's a quickly read but inspiring article about the invention and development of the Aheadset, written with the cooperation of the men who were there at a significant turning point in bicycle morphology:
https://cyclingtips.com/2017/08/origins-how-the-aheadset-threadless-headset-changed-bikes-forever/
