TL; DR: Both the UK and the US have good and bad comedy. Also, SNL is not funny.
Another Australian here and I grew up with an English dad and an America-phile mother. You could say I was raised in both churches.
I'll have to say up front that SNL doesn't appeal to me. I've watched bits of SNL from across the years in an attempt to see what the hell it is that people like about it. And I can't see it. Does not make me laugh. Growing up I watched Rowan and Martin's Laugh-In (when it was repeated, I'm not that old!) and I really enjoyed that. I was recently introduced to I Think You Should Leave, a much newer example and again very funny. Astronomy Club is another new example of good sketch comedy. Hell, almost forgot Key and Peele. So it's not something as simple as "Americans do bad sketches." I think it has to do with how SNL is written. I gather (and please correct me if I'm wrong) that it's mostly done seat of the pants style with scripts being knocked out all very last minute. If so (and again, please correct me if I'm wrong) this "up to the minute-ness" can be very good for a political gag in the moment. But such jokes grow stale fast and other jokes suffer from being rushed and unrehearsed. Also, and I suspect I will cause some offence with this, SNL is so goddamn sure of itself. Every time I watch a bit it gives of this aura of smugness as if it's saying "I'm SNL. Goddam right I'm funny.'' It's not.
British sketch comedy is, I think, overall of a more consistent quality. I can rattle off a bunch of names, many of which have already been mentioned above, but I'll add some classics like Not the Nine O'Clock News, Alas Smith and Jones, League of Gentlemen, Katherine Tate, and Mitchell and Webb.
Re. sitcoms. Both sides of the pond do good sitcoms. And bad sitcoms. The best examples from both are great. And the worst examples from both are terrible. And then there's all that middle of the road stuff. What's that? Some examples? Certainly.
The good stuff:
US: Get Smart, Seinfeld, Curb Your Enthusiasm, Parks and Rec, Community, almost anything Tina Fey has been involved in, Big Mouth, Bojack Horseman, Brooklyn 99, The Good Place, Archer, Rick and Morty, Superstore, Frasier.
UK and Ireland (maybe I shouldn't lump these two together but I feel the sense of humour across the two countries is similar enough that they can be taken together for these purposes. Now that may simply prove I'm an ignorant outsider but I'm going with it anyway.) The Mighty Boosh, The IT Crowd, Detectorists, Father Ted, Derry Girls, The Young Ones, Black Books, The Thick of It, The Goodies.
The Terrible. These threads always seem to talk about the really good stuff. Buy the bad stuff is equally worthy of consideration in a discussion of overall quality.
UK and Ireland: On the Buses, Love Thy Neighbour, Mrs. Brown's Boys.
US: Full House, (anything from the 80's really), Mom, Anything with Aston Kucher, Space Force, The Upshaws, Pretty Smart (I've only seen the trailer for this, and oh my dog, it's so effin terrible.)
And then there's all that middle of the road stuff. Let's be honest, this is where most of TV falls. Friends, Keeping Up Appearances, Big Bang Theory, Men Behaving Badly, How I met Your Mother. This is the sort of stuff one watches on rainy Sundays.
I guess if I have a point it's that I like plenty of shows from both countries.