This command finds the fastest mirror. The contry option is to narrow down the search as well as other options.
reflector -p http -c 'United Kingdom' --threads 2 -f 5 -n 5 --sort rate > /etc/pacman.d/mirrorlist
If you don’t want to disturb mirrorlist file then skip the last part.
reflector -p http -c 'United Kingdom' --threads 2 -f 5 -n 5 --sort rate