With all the different sizes and shapes of bread we have in Belgium "10 slices of brown bread" for the Ginger Jam Bread And Butter Pudding is a rather haphazard way to indicate quantity. Could you please elaborate a bit more?
Also, is it possible to use panettone for this recipe?
Nigella's My Grandmother's Ginger Jam Bread And Butter Pudding (from NIGELLA BITES) is made with regular, ready-sliced (pre-sliced) medium thickness brown bread. The bread is sandwiched with ginger jam and used to make the pudding. You can make bread and butter pudding with panettone, but the combination of the dried fruits and ginger jam here may be a bit too overpowering. Instead, you may like to use a pannetone in Nigella's Italian Christmas Pudding Cake or Panettone French Toast.
In the UK the average weight for a slice of bread is 38g, so you would need a total of 380g of bread. A UK medium slice is around 1.2cm thick and it can be tricky to slice a loaf consistently at this thickness by hand, hence it is easier to use a ready-sliced loaf. UK bread slices do tend to be slightly taller than their European and US counterparts, so you may find that 380g is equivalent to 11 (or even 12) slices instead of 10. If it is 11 slices then just cut one slice in half and use it to make a smaller sandwich.