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There is no prescribed limit (except for mechanically propelled vehicles). Which appears to be the point that TheCatMan and I are both making, albeit in different ways.

It's an offence to be incapable, but, as yet, there is not test for what constitutes incapable. You may not, for example, be compelled to provide a breath / urine test, and it is not an offence to refuse one.

Assuming you are in control of a motorised vehicle you will be arrested for refusing to co-operate with police, or suspicion of driving under the influence and then be taken to the station, where they will throw the book at you for being difficult, and you will be obliged to provide a sample.

The blood alcohol limit is the approved method of establishing your capability, and will be upheld by any court in the land. Everyone responds to alcohol differently, but a line needed to be drawn in the sand in the name of public safety, and I for one fully support these measures despite having been busted myself many years ago.

Having a limit for cycling is legislation overkill. Bicycles are just not such a danger to the public as motor traffic, so I doubt there ever will be a limit, it will be left up to the officer's discretion.