UK Suppliers

Not all the required components can be sourced in the UK, but these links can provide some useful bits. It's expensive getting stuff shipped in from the US, so any components which can be purchased locally are a bonus.

GSM Modules:
Sequoia: They seem to stock the Telit GSM/GPRS modules.

Components
Farnell UK

RS Components

Future Electronics: A distributer which will import things (like the i.MX31 LITEKIT) from the US.