Leave to enter until

Do you think there would be an issue if they flew out of UK on 19th March, then went onwards to Ireland or Berlin, or Madrid for another few weeks, and then got back to the US maybe 2 weeks later?

As long as they leave the UK, it doesn't matter where they go... they just have to be outside of the UK.

However, I would not go to Ireland from the UK, since they would not go through immigration and would be in Ireland on an expired UK visa.

But if they went to any other country other than Ireland, that would be fine .

As long as they leave the UK, it doesn't matter where they go... they just have to be outside of the UK.

However, I would not go to Ireland from the UK, since they would not go through immigration and would be in Ireland on an expired UK visa.

But if they went to any other country other than Ireland, that would be fine .

Thanks. That makes sense now.

As a follow-up to this,  the two Americans went to Ireland, and then returned the following week into Heathrow without issue. One of them stayed for a few weeks and then returned to the US. The other emailed UK Border Force with a complaint, and a few weeks later she received an email with a full apology, stating that he Border Force officer was wrong in their actions, and that she was now free to travel in and out of the UK without issue for the foreseeable future. She has since been in and out of the UK around 4 times.

As an addendum, during their time here, they met up with other Americans and flew into Dublin. They had no issue but their American friends were issued a 5 day visit limit in Ireland.

The embassy doesn't help with visas.  But give them a call if it'll make them feel better. 

I assume they have a few calls about that sort of thing as this is on their site.

"Neither the U.S. Government nor the U.S. Embassy in London can intervene on your behalf if you are denied entry into the U.K., nor can we attempt to influence the U.K. Government’s decision."
https://uk.usembassy.gov/u-s-citizen-services/denied-entry-to-the-uk/

The other emailed UK Border Force with a complaint, and a few weeks later she received an email with a full apology, stating that he Border Force officer was wrong in their actions, and that she was now free to travel in and out of the UK without issue for the foreseeable future. She has since been in and out of the UK around 4 times.

That doesn't mean that she can visit the UK as often as she likes because part of the immigration rules for visitors are V.4.2.(b)
"will not live in the UK for extended periods through frequent or successive visits,"
https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules

There are many posts from those who had been granted a long term visit visa  of 2/5/10 years, who then had their visa cancelled at the border when they try to enter again, for that very reason.

I don't recall any passport checks exiting the UK, so would going through the passport scanner be sufficient to proving you're flying out?

The UK have exit checks and know when people leave or enter. The airlines and ports record this infomation for the Home Office and they also send  advance passenger lists to the Home Office, of those who want to enter or will be leaving the UK. Many EEA citizens who have ordered the details of what the UK hold on them, have been shocked to find the UK recorded each time they entered of leave the UK, as the EU says they must not have their passport stamped. The UK and Ireland also share information.

It's the same with those who have been granted a visit visa and who stayed longer than they claimed they would, but didn't overstay that 6 month visa. UKVI can see they didn't leave when they stated they would as the exit checks show that, and they then get refused when they apply agaiin. They might protest that they didn't overstay the visa, but unless they have a very good reason why they did not leave when they claimed they would and can prove it, they will keep getting refused entry because  they were dishonest. e.g. were in hospital and could not travel on a  plane even with with medical staff, but even then UKVI will check they they have not been using the NHS without paying.

It used to be very common to lie to UKVI/the border officer, show a return flight and then change that flight date when they have been allowed to enter. Too many people abusing, gets that stopped.