I’d agree with njtt, the suggestions will depend on what you like about this one. But, you might consider:
Heresy initially presented by David Baddiel, now by Victoria Coren and since she got hitched to David Mitchell I think her speech patterns are very similar to his! This one is about challenging the accepted ideas - it’s ok. Occasionally properly funny, but it’s not a classic.
I’ve Never Seen Star Wars not a panel game, this one is presented by Marcus Brigstock. I like it a lot - a single guest each week is given a handful of things to do that they’ve never done before. They report back on how the experience went. It’s usually entertaining and contains occasional moments of real tenderness. It also doesn’t seem to depend too much on the guest’s personality - I’ve heard versions where the guest was someone I didn’t really know, or didn’t really like, and it still worked.
Museum of Curiosity guests try to get interesting objects into a virtual museum. Included for completeness - I hate, loathe and detest this one, but maybe YMMV.
Of the classics mentioned above, Just a Minute is probably closer to Unbelievable Truth, but I much prefer I’m Sorry I Haven’t a Clue. I’ve only(!) been listening to Clue for about 15 years, but the running jokes are repeated so very often that you soon pick them up. It has changed a bit since the death of the original host, but I think his replacement is a worthy successor and it still makes me snerk with laughter at least a couple of times a show. Some people do hate it with a passion, though.
If you want to branch out further from panel games, Radio4 does do some good pseudo stand up shows too. Look for Mark Steele, Mark Thomas and Jeremy Hardy and a variety of self-titled vehicles. Be aware that I’m an unrecontstructed leftie though, and those three are the ex-Trotskyite Triumvirate :). They’re not that political these days, but they are amongst the reasons Radio4 comedy sometimes gets accused of left-wing bias. Obviously that adds to their appeal for me, but it may not for you.
My Teenage Diary is a bit of a left-field suggestion too, but it’s both funny and moving. Basically a minor celeb reads out extracts from their teenage diaries, and discusses the context. It’s moved me to tears on more than one occasion. Again, it doesn’t seem to matter who the subject is.
Agree wholeheartedly with the suggestion to check out the Radio4Extra schedules. Radio4 itself runs something every weekday in the comedy slot of 6.30 pm, but Radio4Extra holds all the archive stuff in addition.