Firstly the ACU sticker is for motorsport apporoval only and has no bearing on its legality with regards to being used on the road. If you want to remove it, the crack on, the only approval required for road use is either BS6658-85 (which applies now to older helmets) or the current standard EC22/05 and in any case these standards are usually visible inside the helmet as well.
In respect of the visor, there are two issue, legal tint visor and illegal black visor.
Legal tint visor which must allow at least 50% of ambient light to pass through is perfectly legal to wear day or night, although not advisable to wear during the hours of darkness. To be a legal visor it will be stamped with BS4110 XA, YA or ZA.
Illegal black visor only allows 18% ambient light to pass through, and is illegal at anytime, although it is one of the socially accepted offences as it is more to do with safety than anything else if worn in the right conditions.
Therefore if you get a jobsworth copper day or night he could give you a ticket and in some circumstances it has been known for the copper to seize the visor as evidence, as there is the option of reporting for summons meaning a court appearance rather than a ticket.
That said, most of my former colleagues take a pragmatic view that if a black visor is being worn sensibly for the right reasons, they will let it go as many wear black visors themselves when on their own bikes, not to mention a few I know who wear them whilst on duty.
I know you asked about daytime use, but many often ask me about night time as well which is why I thought I would mention it also.