Links to 7 cajon makers in the UK:

YouTube playlist of cajons made in the UK:
Note that not all manufacturers have current YouTube videos. The list on this page includes websites and Facebook pages which may contain videos not on YouTube. The list on this page is current and actively maintained, the YouTube playlist is less frequently updated.

One photo per United Kingdom cajon manufacturer in Pinterest:

Listed above are links to the cajon drum builders in the United Kingdom as retrieved by Internet searches from March to June 2018. I am sure there are many others not listed. Please send me updates and correction if you have additional information. Thanks.