I had the same problem as you have with the Coopers being K rated, but guess what...
I asked cooper tires why they make them K rated and got this reply :
Cooper Tyres have recently changed the speed rating for some of their off road tyres. This is to comply with the EU Regulation 117 regarding Tyre labelling for Professional Off Road Tyres (P.O.R.).
Tyres must have a max speed rating of K if:
Tread depth equal or greater than 16mm
and
Void to fill ration equal or greater than 35% .
and
Tyres are C3 category tyre (Load rating greater than 121).
or
Noise level is greater than the current EU regulation
Below is a more detailed explanation and an extract from Reg 117
EU Regulation 117. Tyre Labelling: P.O.R
The text below is extracted from page 18 of EU Reg 117 and describes the characteristics needed for a tyre to be classified as P.O.R (Professional Off Road) and thus exempt from Reg 117 tyre labelling requirements.
In addition to Tread Depth and Void Ratios minimum requirements, it also specifies:-
Maximum Speed Rating of Q for C1 & C2 tyres in paragraph 6.7 (a) (iii)
(C1 = Single load PCR, C2 = Dual Load up to 121).
Maximum Speed Rating of K for C3 tyres in paragraph 6.7 (b) (iii)
(C3 = Dual Load greater than 121)
Soooo, long story short, EU makes tyre manufacturers rate them K because they have higher load index than 121 (they are 127, 35x12.5R15 are 113, and thus Q rated), but technically they are perfectly able to run 150km/h.