Sitting in lane 2 when lane 1 is free is idiotic Actually, it's illegal. It's called middle lane hogging, and it's an offence under the broader term of 'careless driving', which can attract a fine and 3 points on your licence.
If there is one, maybe two cars driving in the middle lane at 55-60 but it's clear in front, should a car approaching them in the left lane driving at the speed limit (or maybe just under)then pull all the way over to the right to go past them before moving back over to the left?
No. You should not encourage poor drivers who are committing a traffic offence. You continue in your own lane.
It really worries me that a lot of people don't know what overtaking actually means.
If OP had been driving in the middle lane behind the cars she mentions, and then pulled into the inside lane to overtake them on their left before moving out again to the middle lane, that is overtaking on the left (or undertaking, in common parlance).
That is NOT what she did. She pulled over, correctly, to the inside lane, after overtaking a lorry and then continued in her lane, driving at 5mph under the national speed limit. She was driving correctly. It was those in the middle lane who were driving inconsiderately and, potentially, illegally.