No, but from an imported vehicle/ADR perspective.
Not from a mechanically sound perspective.

ADRs don't specify brake sizes. What is legal is what the average mug inspector thinks is "roadworthy".

It's like this thing about disc brakes and 1275 engines. Is there a book somewhere with these rules? I doubt it.

How did you come to that conclusion, Im in no position to interpret ADRs but "..Any replacement brakes must be either transferred from the original vehicle or be identical to those fitted on the original vehicle" -Motor Vehicle Standards Act. Sounds reasonable to me?

(While reading that apparently cars also have to have self adjusting brakes since 89 which the rovers don't on the rear as far as I know?)

and to MiniMad you where pretty quick to report him on ebay, Lets hope the seller doesn't read this public forum and report your questionably legal brake modifications as payback.

Just my 2 cents, It in no way should be taken as fact without consultation with the relevant authority or a recognised engineer.

Yeah, so what about Camira conversions and the like? Yeah.

I reckon you should all put 1275s in your 850s with single leading shoe drum brakes.

That's my official opinion as a dude on the internet.

The last Camira disc conversion I saw had bits welded onto the Mini's CAST IRON drum type swivel hub. Illegal, by any measure.
Not that Honda conversions avoid scrutiny either, you really have to butcher the caliper and the pad bracket to fit them under any 10" wheels. Then, butcher the inner pad to fit.... and fit 1" spacers too.
I doubt an `engineer' would `engineer' them.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.