According to this article in the Scotsman there have been various ideas thrown about with one being Ascension Island but that it has already been discounted. Other things that could be considered are seemingly cruise ships. Also incredible enough as it sounds even oil platforms were considered a possible option but industry experts discounted the idea at an early stage over safety. It seems they are throwing ideas about rather than all these being actual solid plans. Another option which Patel has refused to rule out is using Scottish islands which would cause a stramash here as the Scottish gvt are not in support. https://www.scotsman.com/news/politics/nicola-sturgeon-responds-reports-uk-government-plans-use-scottish-island-refugee-processi