I don’t think the BBC was using U-matic for broadcast purposes.
U-matic was a common broadcast videotape standard before Betacam came along in 1982. I suppose it could have been Quad (Ampex Quadruplex), which allegedly had excellent picture quality, but those were massive reels of 2" wide tape that cost about $300 each (for 1 hour of recording time), and as such, recordings were rarely saved; they were typically recorded over and over again, as many times as they could. The tape reels were so wide and large because the recording heads didn’t use helical scan, which is what allowed high-quality, yet compact, videocassettes to be possible.
The site you linked to seems to only have information on the restoration of Dr. Who episodes which exist in oddball formats. For example:
Back in 1978, Ian Levine heard that KCET TV in Los Angeles was about to show `The Dæmons’ as a two-hour compilation. He wired an American friend the money to go out and hire one of the then brand-new Betamax VCR’s, and to buy two one-hour tapes, at that time the longest tape available. The machine was obtained, and the broadcast recorded in its entirety, except for a gap of about twenty seconds during which the tapes were changed over. The Betamax tapes were brought over to England and, as TV standards conversion equipment was not generally available to the public, the tapes were copied onto 525-line U-matic cassettes, retained by Levine ever since.
So in that case it was a recording of an over-the-air NTSC broadcast onto Betamax (which was consumer-grade junk to begin with, compared to broadcast-quality formats such as U-matic and Betacam), and then dubbed to a U-matic tape, still in NTSC format.
The Mark Hamill video seems to be significantly higher quality than U-matic or even Betacam is capable of (even allowing for the extra 20% lines of resolution of PAL vs. NTSC), so I’m guessing it was Quad (though it seems strange that they would have saved the recording considering the size and expense of those reels). I’ve never seen a Quad transfer, but I’ve read that the high-band and super high-band versions from the '60s and '70s had “excellent” picture quality. The TV broadcast of The Nutcracker (1977) was on Quad, and was preserved, and is available on DVD and Blu-ray, so that would give an idea of what Quad was capable of. I’m not even remotely interested in buying it though.