Um. 10 x 5 + 10 x 10 + 10 x 20 = £350
Um ... don't the Jocks still have £1 notes?
That doesn't give us an explanation of £105, but it does hint at how one might look for it.
For your bonus question, why did I regularly pay between £10000 and £20000 for a pizza in the 1990s?