With Paddington, and other terminal stations, you also have the issue of there being trains which aren't announced yet, but due to tools like realtimetrains, people can work out which train their train will be - I tend to never physically go onto the platform before the train is called, since it will just cause confusion for the staff and as long as you position yourself near the correct platform, you'll always get a good seat, but somewhere like that I imagine the policy is always to never let someone through until their train's platform has been announced
I've had a member of staff let me through in exactly those circumstances from the bridge gateline - I said something along the lines of "It's that train, I know it's that train, I can see it says on the side of it, better that I stand out of the way down there than wait here in the way, I won't get on until it appears on the displays" and he let me through. And I indeed did what I said I would, stood under the stairs out of the way and didn't board until it came up on the screen. This was on a paper ticket, though, it just wasn't encoded properly, one of the TVMs at Bletchley is a bit iffy with encoding tickets, I just always forget which of the two I used by the time I discover it has happened at Euston (Bletchley gateline is almost never in use now).
On the main gateline they usually won't, though.
We may well reach a point where gatelines and the PIS are integrated well enough that an Advance ticket won't be let through until the booked train shows as boarding, but we're a way away from that. And it really depends on the station as to whether there's a reason you may need to get through ASAP - New St is arguably one because of the toilets (there's very little retail inside that's not duplicated outside), but as mentioned Paddington and Euston have nothing inside bar trains, and somewhere like Temple Meads has everything inside bar the booking office, as would Preston if it ever gets a gateline.