Yes, they were. As far as I know.
In fact, one surfaces on our "Williams Studio Animated Ads" DVDs. (Although the tape I got it from didn't concretely credit it to the Williams studio, it seems to be the studio's work.) It features the white rabbit from Alice in Wonderland.
Some good evidence of that is, as you said, the use of Miriam Margoyles for voicework.
"I need a change of pace, I need ... bunny sex." - Clifford Menigee, Gods of Los Angeles
"Got a thing for rabbits, huh?" - Eddie Valiant, Who Framed Roger Rabbit?
That Corona one that you identify as Dick's does look it - it has, in those simple designs, an obsessive attention to perspective typical of Dick.
I've watched the others and I'm not sure. Since pretty much everyone working in animation in London around that time had trained with Dick, it would be hard to tell a Dick commercial on sight from one by, say, Richard Purdum or Tony White or Uli Meyer.
Still.
The Um Bongo ad has kind of a similar movement to an ad starring some monkeys, that is on the Animated Ads disc and which I believe is by Dick.
It's also similar to some of Oscar Grillo's work. So, could be his work. Could be anyone's.
The United Ad reminds me a bit of some of Dick's work in the perspective changes, and the line style could compare to Dick's Tic Tac ads.
The bear in the Sweetex ad moves similarly to the Disney characters in Dick's Fanta ads.
Who knows.