Thanks! It seems that opening a new Incognito window in Google Chrome also works*. However, it's unfortunate that Eurostar don't provide a more convenient way of changing sites (especially as it seems you can't book one-way France->UK travel on the UK site, nor vice versa - so you need to swap sites in order to book such travel).

It also seems that the US site only lists a limited number of stations in France (while the UK and French sites offer a wide number of stations in France, with connections by TGV).

Also, although the UK site will sell tickets from London to Switzerland (with connections on the TGV from Paris), I can't see any way of booking a one-way ticket from Switzerland to London. (However, such tickets can be booked on sbb.ch - and presumably also on www.voyages-sncf.com...)

* Presumably because Chrome handles cookies separately for Incognito windows. This method saves you having to manually clear all cookies.