Basic lack of a multiple threat where we need to engage with interceptors, medium range SAM and Shorad.
Medium range essentially static SAM are still point defence weapons. We limited deployment to protect our nuclear bomber assets in UK, Cyprus and Germany. We also had Bloodhound in Singapore
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With more money we could have deployed SAM around other key installations and cities. The USA had lots of Nike sites but these did not give blanket coverage. SAM belts cost $$$$$ and were really limited to the USSR border and the central region.
Given the small size of our land/air forces the cost of modern long/medium range SAM would be wholly disproportionate to the threat.