Efficiency chosen over price!
The Samsung kit is significantly more expensive than many competing solutions, but is less power-hungry, much more reliable, and requires much less maintenance. Three will build their network up somewhat then sell it to one of the competitors as a "going concern".
EE have significant Quality of Service problems, so their 4G offering is likely to be rather less than spectacular. Since the network amalgamation they have laid off over 60% of their engineers, 40% of their call centre staff (which they've entirely moved abroad), and they are beginning to "rationalise" their retail outlets, closing either T-Mobile or Orange branches in each location - survival depends upon branch turnover.
Britain no longer has any network capable of providing comprehensive services:
O2 are hamstrung by their older equipment, poor site selection and traditionally high prices.
Vodafone have major capacity problems because they completely misjudged the take-up of data services.
EE are just entirely incompetent. Shaving staff, allowing the major failure of swathes of their network without repairs (coverage in parts of London and Birmingham is a joke), and a billing department that loves to overcharge or make multiple charges seemingly at random.... their ineptitude is stunning.
Meanwhile, Britain is the second most expensive place IN THE WORLD to make a phone call!