Windscale Uranium Processing Plant may have had some imported sirens, but that wasn't until after the accident there. My gut feeling is that there were standard Secomak (Klaxon Signals) sirens there instead of any others.
Also in the 1990s, the Civil Defence Movement paperwork was scrapped by the Conservative government, and subsequently the nationwide siren system in the UK was officially decommissioned, and the sirens removed and sold/donated to flood zones/ given to war museums/ restored for display.