Anyone with Sky Broadband Shield is unable to play Halo 5 online or sync account before 9pm. This is a shocking state of affairs. No other game i know of is affected this way including 18 rated games like Gears Of War, which i've tested.
The current experience you are referring to above, have been reported by other users. Please take notice that it is being investigated as we speak. However, you would to be able bypass the filter by allowing multiplayer.
Please stay tuned and keep an eye out for future updates.