So I have trawled the internet and forums to see if anyone has fitted 285/55R20's

It seems 275/55R20, is quite commonly used with none to minimal rubbing.

However step up to 285/55R20 and you get a nice selection of LT tyres from both Goodyear and Pirelli....

And at only 6mm bigger radius, it looks pretty close to fitting to me, maybe a minor cut out at the front wheel liner and the usual fuel filler cover mod.

Anybody anywhere used this size?

I have a set of 285/50/20 Cooper AT on std 20” L405 offset wheels, they catch ever so slightly on full lock - not much just a very faint rub. Therefore the 285/55’s are definitely going to rub, unless you can adjust any steering lock stops to give a reduced turning circle - I’m not even sure the L405 has them like the old RR classic and Defender 2021 L405 Vogue SE 4.4 V8 DIESEL ~ #17

But 285/50R20's are smaller (793mm vs 811mm) in diameter than a 275/55R20 which people often fit with minor rubbing similar to your description, I can't imagine that 5mm further out will outweigh the 9mm increase in sidewall bewtween 285/50 and 275/55

I'm not saying your tyres don't rub! lol
Just trying to justify my maths that the 285/55 is again another 6mm more sidewall and 5mm further out than the 275/55!!

I don't mind trimming the inner wheel arch at the front if required.

It’s not just 5mm further out though - they are also 5mm further in on the inner wall, and it’s the inner wall that catches.

Buy a set and try them  2021 L405 Vogue SE 4.4 V8 DIESEL ~ #17