OK, you'll either love it or hate it.
You can get 4 small 125g jars of marmite for £3.87 delivered at amazon, by signing up for delivery at regular intervals. You do not have to honour the regular delivery, you can cancel once ordered. I usually go for "every six months" then cancel once it has been delivered.
This works out at £7.74 per kilo.
You can also get bigger 250g jars (4 jars for £8.05), but this equates to £8.05 / kg,
The cheapest you can get this at tescos is £10 per kilo or more, depending on the jar size.