Diolch!
I almost gave hope but when likes came in I’ve seen that both, Sir Terfel and his son liked my tweet with the question so the little “investigation” could go on. Now I’ve found and followed both - Sir T. Jones and Tomos Terfel. It could be interesting. 
Da iswn Tatjana fach!
Diolch @henddraig.
I actually did a bit of investigation if one of our magazines woud or might plan to get an interview or something like this about the visit or something like that, but they said one another media house covered the event with pictures and all so they’re not interested in doing so (or they perhaps mustn’t be interested due to some unknown to us rules).
@tatjana, on youtube you can find The Last Night of the Proms, 1994. The second half of the concert generally follows the same pattern, and in 1994 Bryn Terfel was the singer playing the principal role. He sings Rule Britannia, including a verse in Welsh. You may enjoy it.
Oh Margaret DIOLCH YN FAWR IAWN IAWN!!! I don’t know how I came to have never heard about this. I didn’t watch the Last Night of the Proms because it was so very English (usuallly) or even know he was singing, but I saw nothing about it afterwards. I was living on Gower. I wonder if I was at a Star Trek Convention? Oh well, it may have taken 23 years 2 months and a few days, but thanks to you, I have seen and heard it now! When he switched to Welsh, I was crying! We, after all, were here when the Romans adapted a Greek name and called our island Britannia!
Diolch @margaretnock! You couldn’t give me the better to watch. I love how he came in rugby shirt all Welsh whatever he wore - the dragon, the flag, the rugby ball … WOW! Sir Bryn Terfel thanks to your son and Postojna cave you’ve just got yourself a fan in little country Slovenia!
And … seing another video clip of him singing Hen Gwlad I I’ve realized I’ve learnt to sing it by his recording - exactly the video I’ve watched just right now.
O gosh, I am a lucky person! Diolch, diolch, diolch!!!
O boy … it only now occured to me what I actually did! . …
When Postojna cave staff has got into mix of conversation inbetween me and Sir Terfel, saying that they’ve understood everything with the help of Google Translate and I’ve envited them to learn Cymraeg with us I also pointed out that I was there in September with another Welsh learner and that I even sang in the concert hall presenting them with the clip of my singing not to be aware I’ve (of course) presented that same clip in my answer (kind of nonchalantly obviously) to the famous opera singer as Sir Terfel is, too … If he listened to that, he surely might feel aweful due to my unproffessional singing and my bad pronounciation of Cymraeg, but it didn’t come to my mind what I’m actually doing at the time. …
But, well, that’s how life is …
Did Sir Bryn mention your singing in his reply? 
No. There was no reply from him anymore, what can tell something too. 
I can imagine him saying “Owowow … what a bad singing …”
The strange thing is I don’t even feel bad about my putting my singins to view, but it rather amuses me for some reason.
You are a much better singer than me, and very brave to contact the Mighty Bryn. (Music dad joke).