Second homeI would like to add some comments to this interesting discussion, as regards UK capital gains tax when in due course the second home in the UK is sold.
The tax position at that stage depends on whether or not Luvbritain is UK resident for tax purposes.
Tax as a non-residentIf Luvbritain is not a UK resident then any again will be subject to UK capital gains tax. Until recently the general UK position was that it did not tax gains of non-residents, even in respect of UK situs property. This general position has however been changed in respect of UK situs residential property. This first change in this area was to charge tax on UK residential property which was in the ATED rules. This will not affect him. However from 6 April 2015 all non-residents are liable to UK capital gains tax on UK residential property, referred to as NRCGT. He can see more details about this for instance here-
https://www.gov.uk/capital-gains-tax-for-non-residents-uk-residential-property
Tax as a residentIf Luvbritain is a UK resident then any gain would be chargeable tax to UK capital gains tax, subject to the principal private residence rules. Since Luvbritain would have two residences the main aspect to note is the election which applies in these circumstances. The ppr election rules were amended in 2015 so that it is only possible to elect for a residence to be a ppr when the 90 day test is satisfied. Again, there is more detail available on HMRC website.
Other pointsThe UK US Double Tax treaty does not prevent the UK from taxing gains on UK real property. My reading of the treaty is that the UK has first taxing rights and that the US is obliged to give credit for UK tax against US tax.
I would be interested to hear the view of Luvbritain and others. I think that this topic will become more common.