I’ve replied to this before, but it’s quite simple.
I’m a member of a club (well, an informal group, really) which meets periodically to hold running days for their locos and rolling stock. 20v AC power supply has long been extinct in the U.K., apart from a small number of Lionel and older Märklin and Hornby locos which are still in use. There are a number of minor O Gauge suppliers like Daersted and Dapol, and quite a few BL locos. 3 rail generally is very much a minority interest here.
Nobody apart from a small number of Lionel specialists uses the TMCC systems, which simply aren’t available here except by private import.
There is a general practice of running 20v AC locos on 12v DC. I took along an old Scout, and a modern Polar Express Berkshire and experienced no problems at all. There were at least two Lionel Halls (Hogwarts Express), the day I was there. For running days, the issue of reversing from the transformer isn’t a problem, because the main purpose is to have some running time on a big track; as long as the loco will run forwards, that’s enough. Tender whistles are just disconnected.
The K Line loco caused much comment, because no one had seen one before. The wire tether caused much comment. I don’t know how it came to be in the U.K.; I bought it for under $100 equivalent in eBay on its second re-listing.
As far as I can understand, because electronics isn’t my field, it ought to run without difficulty on 12v DC - but I can’t seem to confirm this. (The literature says the Berkshire won’t accept 12v, but this is clearly not the case - I can only assume that this is a case of manufacturers literature rejecting anything outside their own system, not uncommon). The whistle can be isolated by disconnecting the pickups from the tender rollers. AFAIK, it doesn’t have TMCC or equivalent.
I’d like to give this rather fine model, a good run out with a 4 or 5 car train of full length stock, which I have no opportunity to do at home. I think it would be well received at a running day. However I’m reluctant to take unnecessary risks, and I’m reluctant to spend £300 or so on a full conversion which would have little, if any resale value.