This is sounding like if you move mid year you will just have to "suck it up" and pay the ACA fine for the balance of the moving year if you drop your insurance.
The "penalty" will not come about until one files a tax return.
If an election is made to use form 1116 for the first partial year, and not form 2555, then the penalty may well come into force.
On the other hand, if the election to file a Section 911 exclusion is used (form 2555) and the taxpayer applies for an extension to the filing date until they will qualify for the exclusion, then there will be no penalty due. At the time of filing, they will qualify. If the taxpayer moved to the UK in, say, May (Year 1), by 15 June (the automatic extension date for those resident outside the US) of the following year (Year 2) they will have easily met the 911 requirements. An extension may be requested to delay filing until October without difficulty. For those who move after October, I believe there is an allowance of 3 months for non-coverage available. I may be well wrong on the last point. The only problem arises if the taxpayer owes tax for the partial year (Year 1). In that case, a payment for projected tax due should be made before 15 April (of Year 2) to avoid "late payment" penalties for Year 1. Fun and games.