Not sure this is what you're talking about but years ago there was a period of interest in using the P40 drop-in modules in the SL4 and SL6 lights. Is that what you're referring to? That module is smaller OD than the SL4 requires and a metal spacer is needed to complete contact. There were a variety of simple DIY solutions to that need described in various forums. I never saw a direct-fit after-market replacement at that time, or the last time I checked one of the prominent vendors, DX Extreme. I'm not familiar with the Halcyon Scout application, but in an SL4 this mod produces a powerful long-lasting and inexpensive tight-throw light.
Those P40 modules are still available, with some selection of emitters and drivers, last I checked. Cost recently ran around $10-$12.
I think the guy who was packaging those in ready-to-use form may have been jblomquist, or a handle close to that. Don't know if he's still active.
It's been a year or so since I've looked into this, so if this is off the mark, my apologies. $85 for that UK part is absurd.