Hello,
the task looks interesting, I would like to do it for you, work out all stages - circuit, pcb, firmware, manufacturing. Unfortunately I am unable to come to London in the next 2-3 month, hopefully we can also exchange per skype.
About the time line: as I did understand from your description the size of project, it will require in total ca. 20-30 hours of work to be complete, getting a small count (5-10) of ready manufactured devices. I hope, it meets your expectation. Please contact me on interest, it would be fine to draw up a plan for the development.
Best regards,
Viktor Gotwig,
Cologne, Germany.