Okay, we have an update, which won't surprise regular followers of this thread.
So in the tortoise versus tortoise race between Gunfire and Taiwangun to import a RIF into the UK, team Taiwangun seem to have fallen at the first hurdle, and to quote an email they sent me last night, "...for the moment we are still not able to sent the package. We are working with DHL but is not so simple" [sic]. So in short we haven't even got to a place where getting TG to import into the UK is a realistic prospect.
Team Gunfire, however, did manage to send their parcel that I ordered on the 19th December on the 21st December, after which we enter the wonderful world of UPS again. To be fair to UPS things have improved slightly, insofar as they managed to get the parcel to the UK on the 23rd December, but unfortunately there was no Christmas miracle. On the 28th December I received an email telling me that UPS were delivering the parcel (with apparently no RIF declaration required), but the cynic in me said that this was unlikely to happen, and it didn't. An import scan took place on the 4th January, and on the 5th January UPS rang me to tell me that unless I filled in a RIF form by 1400 that day they'd be sending the package back to Poland. I pointed out that this was the first time they'd spoken to me and I'd received no emails or phone calls to ask me to fill in a form. The agent then emailed me a form to fill in, which amused me somewhat as it contained a packing list that included cake, cheese, cookies, soup, fudge, a handwarmer and some waffles, rather than a RIF and some kit. Anyway I sent UPS an old form I had which I amended with the new order, they billed me the import charges which worked out at about £60 for a £300 order (20%-ish?), plus a £10 processing fee.
The parcel is currently sat awaiting Customs clearance, but is getting scanned every other day. Knowing UPS they'll update the tracking after the parcel is on my doorstep, so it's anyone's guess when it'll be released and delivered; I'll reserve judgement on how well they've done after the parcel turns up.
Lessons learned? TG is still a no-go. Clearly using DHL is not working out as they thought. Gunfire is a possibility so long as you don't need anything too quickly and you meet their minimum order threshold. UPS are a lottery as usual, but at least their UK brokerage team called me (eventually), and they themselves said they think things are better now, compared to 12-months ago. I'll update again when the Gunfire parcel arrives, so you can judge the delivery times yourself and whether it's worth the inevitable hassle.