Nice work - always good to see big British guns!
The white paint under the barrel was to help break up the straight line of the barrel - the lower side would naturally be in shadow, and hence dark. By painting it white it helped disguise it. You can see similar on a lot of animals - they have paler undersides.
This was also used in WWII on Sherman Fireflys to help disguise the much longer gun over the normal Shermans.