As Guido points out, 100% of Marmite is produced in Burton using ingredients sourced entirely in the UK. A weak pound makes Marmite cheaper to sell abroad, this is an attempt to rip off consumers using Brexit as an excuse. Unilever, of course, was part of Project Fear.
Tesco have refused to pay Unilever's unjustified price increases and have even gone as far as withdrawing some of their products. Tesco makes their own version of Marmite so it's not like their customers are going to have to go without.