UK timezone confusion

A client of mine has the site’s timezone to Europe/London. Events whose times that are set in the future and during British Summer Time are shown as one hour earlier. Not sure why.

Currently, I am displaying the event through a third party plugin that allows me to have load more functionality. I retrieve the date time like so:

$start_date = get_post_meta( $post->ID, '_event_start', true );
$start_date = date("j M Y @ g:i a", strtotime($start_date));

Am I doing something wrong? Setting the event timezone to GMT works fine, yet I am not sure if this is the correct thing to do. I am quite confused, can you help?