Could the shield be one of the Home Office's investigations to prevent wildlife (as wildlife could block the rotors - birds, etc.). They experminented with various covers (such as protective Glass-Reinforced Plastic shields) but found that these reduced overall sound levels by between 5 and 8 db, finding these methods unsatisfactory. It was then subsequently realised that 'a simple wire-mesh cover would suffice to prevent ingress by wildlife (such as the Portland, Uk, flood warning sirens). Preliminary trials of such covers showed that they produced an attenuation of only about 1 db, which was considered acceptable'. I believe a similar shield to Andrew Pollard's photo on a siren is featured in the UKWMO booklet published in 1985.
Also, on the subject of:
I've noticed that many extant British installations are in what American engineers would consider odd places, such as on low pole mountings or under the eave of a building. Many WWII installations were on poles shorter than the surrounding buildings, not much taller than a lamppost. American manufacturers of horizontal sirens relied on the refraction of sound off of surrounding structures (though they generally preferred taller poles and rooftops), while modern installations of vertical sirens try to aim the source directly at the hearer. Perhaps the British were following the former example.
.. British manufacturers of sirens appeared to suggest lower mountings- but nearby to large open spaces etc. (such as on the corner of parks), as can be seen by this 'Secomak' publicity quote "sirens should not be mounted too high above gound level; 4.5 to 6m is usually recommended. Putting sirens on top of high buildings often has the effect of deflecting the sound waves upwards because of negative temperature gradients. Sirens should not be located close to tall buildings. Ideally there should be at least a clear 50 m radius around each instrument."
Though, this appears to contradict the Home Office's Study Group into sirens (1992) which quotes the FEMA guidance of '...recommends a minimum mounting height of 15 m in low-rise areas, partially to assist the sound propagation, and also to reduce the risk of startle and possibly even ear damage to people near a siren when it was activated. In high-rise areas, it is essential to mount the sirens above the rooftops of the surrounding buildings, and when they are mounted on a parapet roof, they should be at least 3m above the parapet. If these criteria are not satisfied, the coverage is drastically reduced. It was found that for the majority of the (UK) sirens, these criteria were not satisfied'.
However, by 1992 (sorry UK siren manufacturers), the Home Office recognised that 'The design of the exisng power siren system was based on unrealistic claims by the siren manufacturers for the effective ranges of their equipment'.