Possibly the few Valjoux's from avi-8 and Matthey-Tissot ? Although they were overpriced at £1.5k+ to begin with compared to USA/online you pointed me to. Do they increase or at least stay static? And think somebody mentioned they sold an Omega or other proper mid end brand, tag or other on IW at some point? Are Omegas a good investment. What watch is?
Don't remember them ever having Omegas, although an omega did feature in quite a few shows despite Mason's valiant efforts to keep it hidden.
In fact, we've had quite a few big Swiss brands on IW, just not for sale, Simon Iles - Rolex Yachtmaster II, David Fabro - Rolex Submariner, Peter V - Rolex Submariner, Shaun Crawley - Longines Hydroconquest, Sally - Cartier Santos, Den - Cartier Santos, Shona - Cartier Santos.
The only brand they have had on a show was Eterna, the few Tags they had only featured for a short time on their web site.
Gordon may have purchased an Eterna, but pretty sure the tags were already gone by the time he started collecting in 2020, they briefly appeared shortly after James Martin took over in 2018.
As to keeping value, Omegas will certainly keep their value better than anything that's appeared on IW, but even Omegas lose their value, just not as quickly. Very, very few watches keep their value or increase in valve. And of course another problem is, as you've alluded to, if you've bought your watch at a higher price too start with, then you see an even bigger drop in re-sale value.