Surely you can just go if you want to ? Honestly, no one thinks it is odd to go as long as it is not before the ceremony ends !
My DD got married last year - married in same venue as reception, ceremony 2.30 , drinks and chatting with jazz, then meal,,speeches etc, then dancing and cake/cheese board. People left at various times , as expected - some directly after meal, some about 9, we left at 10,30 with most of the older generation, the couple left at 11.30, which coincided with the ( free ) bar shutting, then everyone was rounded up by 12, which was the end time set by the venue,
It was long enough - it had been a very long day for everyone, lots of talking, lots of drinking. If people wanted to have after parties elsewhere they could have done - this is London - but I would hate to think that anyone would have been hanging on, unhappily, because they felt they should for some reason.
I’ve been at weddings in remote hotels where you can be more ‘trapped’ - slope off, and let earplugs be your friend .