Adam â€” I think they are unlikely to be of British manufacture, unless they were specially commissioned for a British factor, wholesaler or retail group. I prefer a mainland European attribution, but would not be too surprised if they turned out to originate from the USA.
Your best hope of a positive attribution is if they were also made in carnival glass, as dealers and collectors of this style have immense knowledge of C20 unmarked pressed glass.
As for date, it is difficult to help. All those I have seen have been in good condition, typical of, say, 1930s or post-war pieces. I have in mind a lovely range of "Victorian" epergnes which I was shown on Murano last year, which were launched on to the English market during the 1950s!