I do think the casting is pretty good - they're just not getting the most out of them. 8 weeks would work, even with 16 housemates - a couple of double evictions along the way, a surprise eviction (live, not overnight) and an additional double eviction in the final week gets us down to 4 for the final.
For me it needed a couple of new hooks. I do understand why they started with a clean slate, so picking up the Big Coin idea 5 years on wouldn't have been wise. I've said it before but one thing which really worked for Nine was the nominations superpower - gives you a simple twist every week and would give them the chance to do a weekend task with purpose. There are then multiple things you can do with it, but honesty is crucial so viewers need to know upfront what the power is - so either as the US did release details of them all at the beginning or perhaps have a poll each week where viewers pick between two powers.
The other thing I don't think works now is the weekly task - just no jeopardy in it. Therefore although I'd keep the structure of the weekly task I'd make it a Have/Have Not task. Repurpose the bedroom and bathroom so the bedroom becomes a luxury bedroom with luxury en-suite then the bathroom becomes a Have Not room with not so luxur en-suite.
These are both quite simple, unoriginal, twists - but something I think would really work with BBUK at the moment and just lift it a bit from just being a bog standard series without compromising the more back to basics approach.