Modest meal in a pub + drinks (soft not much cheaper than booze!) for four (and the kids always want dessert) can easily be ?50. Coffee/juice & cake for tea or elevenses, say ?2 for drinks and ?2 - ?3 for bun etc. x 4 there's another ?20. You'd be amazed how quickly it racks up with a family of four - actually, now I think about it, ?150 is really a very modest estimate!
The point is, you only have elevenses because you can afford them - if you couldn't, you wouldn't - you'd just go without and I'm sure you'd be ok for it.
I can see your point, and I know what you mean - it is amazing how things add up, but it's also amazing how much you can do without if you really have to. The trouble is that you don't tend to do without unless you really have to...