I think people will care if emigrants start to trickle, and then maybe even flood, home.
For the people abroad on sterling based incomes coming home may start to look ever more attractive. Their incoming funds have effectively dropped like a stone BUT if they release euro based capital (e.g. sell the EU house they live in) and whip the proceeds back the UK while the exchange rate is this low, or even lower... depending on what the exchange rate was when they left, they could make a significant profit.
I think (hope?) that the Uk and the EU will bash out a deal that leaves everybody who moved countries pre-Brexit with the right to remain and work. Which I think is the only sane solution. However the plunge of sterling in conjunction with that sort of policy could mean more pressure placed on U.k. housing stock and services if the number of returnees is a lot higher than has been anticipated.
Personally I'd be wanting to keep taking the pulse of the UK emigrant community in Spain. It's huge. Spain is having issues of its own, you add in U.K. based income falling in terms of day to day buying power and it's not beyond the realms of possibility that anything from a notable wave to a Tsunami of people could end up feeling coming home is their only realistic option.
I don't like Marmite and they don't sell it where I live anyway. So not fussed about that. However it's not really about Marmite. It's about the extent to which the UK typical shopping basket might cost a lot more, and what impact that will have on inflation. And/or the risk for British based food industries, with the potential for job losses.
So, I'm not picking either/or as more important. They are both very significant early alarm bells of what might be on the horizon.