The only solution ATM is to check https://www.reddit.com/r/Ubuntu/comments/3q53kc/list_of_ubuntu_repository_mirrors_available_over/ an chose a nearby mirror.

Then compare http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease and your mirror, e.g. https://ftp.fau.de/ubuntu/dists/bionic-security/InRelease (mirror of the FAU (university) located in Germany) and make sure "Date:" in the top section of the InRelease file is equivalent (not more off than 6 to 12 hrs) between the two files.


Then adjust your /etc/apt/sources.list e.g. like this:

## Main repository for Ubuntu distributions. This line was auto-added in
## 2018 to provide a fallback, in case no network mirror is available.
# deb cdrom:[Xubuntu 18.04.1 LTS _Bionic Beaver_ - Release amd64 (20180725)]/ bionic main restricted
# deb cdrom:[Xubuntu 18.04.1 LTS _Bionic Beaver_ - Release amd64 (20180725)]/ bionic universe
# deb cdrom:[Xubuntu 18.04.1 LTS _Bionic Beaver_ - Release amd64 (20180725)]/ bionic multiverse

## See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
## newer versions of the distribution.
## Major bug fix updates are produced after the final release of the
## distribution.
deb     https://ftp.fau.de/ubuntu/ bionic main restricted
deb-src https://ftp.fau.de/ubuntu/ bionic main restricted
deb     https://ftp.fau.de/ubuntu/ bionic-updates main restricted
deb-src https://ftp.fau.de/ubuntu/ bionic-updates main restricted
deb     https://ftp.fau.de/ubuntu/ bionic-security main restricted
deb-src https://ftp.fau.de/ubuntu/ bionic-security main restricted

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb     https://ftp.fau.de/ubuntu/ bionic-backports main restricted 
deb     https://ftp.fau.de/ubuntu/ bionic-backports universe multiverse

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb     https://ftp.fau.de/ubuntu/ bionic universe
deb-src https://ftp.fau.de/ubuntu/ bionic universe
deb     https://ftp.fau.de/ubuntu/ bionic-updates universe
deb-src https://ftp.fau.de/ubuntu/ bionic-updates universe
deb     https://ftp.fau.de/ubuntu/ bionic-security universe
deb-src https://ftp.fau.de/ubuntu/ bionic-security universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb     https://ftp.fau.de/ubuntu/ bionic multiverse
deb-src https://ftp.fau.de/ubuntu/ bionic multiverse
deb     https://ftp.fau.de/ubuntu/ bionic-updates multiverse
deb-src https://ftp.fau.de/ubuntu/ bionic-updates multiverse
deb     https://ftp.fau.de/ubuntu/ bionic-security multiverse
deb-src https://ftp.fau.de/ubuntu/ bionic-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
## It includes Adobe Flash, Google SDK and IBM Java.
#deb     http://archive.canonical.com/ubuntu bionic partner
#deb-src http://archive.canonical.com/ubuntu bionic partner


Then run:
sudo apt-get check
sudo apt-get update
sudo apt-get upgrade
sudo apt-get clean
sudo apt-get autoclean
sudo apt-get autoremove
sudo dpkg --purge `dpkg --get-selections | grep deinstall | cut -f1`
sudo apt-get check

Make sure `sudo lsof /var/lib/apt/lists/lock` does not output something besides a warning of /run/user/1000/gvfs. If so terminate the offender if possible and/or run `sudo dpkg --configure -a`.


That's it. All your APT updates will now run TLS-secured (HTTPS), -except- for your Launchpad PPA updates, located in /etc/apt/sources.list.d/. AFAIK their links cannot be transformed from HTTP to HTTPS yet.