EMR want bi-mode capability. There aren't currently any bimode locos that fit in the UK's loading gauge, so multiple units are a necessity. MUs are also far more space-efficient than loco setups allowing for more passenger capacity within a given length, accelerate far better and handle poor weather conditions better due to distributed traction and, when it works of course, allow for split services / capacity adjustment by operating in multiple. Loco-hauled trains do have some advantages, but not many. Their time is arguably up for regular passenger services in the UK.