I did get a response from Solvay informing me that the following company is responsible for bicarbonate of soda in the UK:

http://www.brunnermond.com

The Brunner Mond group owns the Magadi soda company of Kenya. Brunner Mond is wholly owned by Tata Chemicals limited

http://www.tatachemicals.com/

So...I emailed Tata to ask what the source for the UK's bicarbonate of soda is and I'm still awaiting their reply.

The saga continues.