As most, if not all , weddings take place in day hours in the UK(before 1700 hrs usually) it is quite in order for guest at the service clad in day wear to "go on to the do" in the evening clad in day attire. Guests arriving later, just for the evening bash, if it is formal(ish), would normally turn up in black tie evening dress.

Ultra, super, mega, smart weddings like the first Royal wedding this year did allow for the guests at the service to rid themselves of heavy tunics, gongs, etc and did dress in evening attire(black tie) for the evening bash as it was more comfortable.