Looks like some very nice and super sturdy beds!
Are these sleepers actually stock lumber in the UK, or is someone milling down actual railway sleepers to get the creosote and tar off them? Here in the States you can get the "raw" (and super filthy!) sleepers which are about 2.6m x 250mm by 150mm or so.
These are stock in the UK. Oak has become a good option lately as it is grown, harvested and milled all in the UK. Treated softwood primarily comes from Scandinavia, and due to various reasons which include the UK leaving the EU, Putins idiotic actions, a continuing backlog due to covid staff shortages across Europe, and a bad winter in Northern Europe, the price of importing timber is crazy right now. Reclaimed imperial railway sleepers are available but they are slowly becoming scarce.