Due to Xiaomi not officially being available in the UK, the only way to purchase is through sites such as Aliexpress or Gearbest.
This will likely cause VAT to be charged when they are being imported, judged by the value declared on the packet.
As a rough rule, anything sent by China Air Mail or similar will have a higher chance of avoiding customs due to incorrect value being declared by the shipper, but this is not guaranteed. However, they are usually also untracked and slow.
DHL, UPS and others often offer < 7 day delivery times, but will charge more for both delivery as well as requiring accurate values are declared. Use this calculator to give you the true cost of your phone, once delivered to the UK. Bare in mind this is a guide only, and the real value may vary based on value declared and shipment chosen. More information on import tax can be found here.