Tell a lie about the Volvos... that's the D5 that eats headgaskets, T6s eat transfer boxes

Thinking it might be a cost issue... the plod BMWs must be stupidly cheap for everyone to be buying them... guess that BMW simply produce an un optioned 530d and make pennies on them

Once upon a time, manufacturers stuck their biggest engine in a basic car, and sold it for a sensible price...
'95 Volvo 850 T5R Police spec, £12,500
'03 Vauxhall Omega 3.2 Estate Special, £18,300
'13 Chevrolet Caprice PPV, $40,000/£25,000 available in rhd too
'13 BMW 530d, £18,000 ish
'13 Volvo V70 D5 £18,000 ish
That still happens now to some degree, but either manufacturers are no longer offering their best products, or Forces aren't allowed to buy them

Even a 535d would be more reliable, as it has more to give, so would perform at 80% load rather than 95+% that the 530d needs to to keep up... same applies to the 550i, sure it would use more fuel, but it would break less often as it would be far less stressed...
Diesel fuel cost savings are far outweighed by down time and repair costs...