For handguns: Much harder for you and me than for a determined criminal, obviously. And then repeat 100 times.
I'll leave the nitty gritty nuts and bolts to those who reside in the UK and have dealt with the system.
From following high end horology forums packed with City of London financial types with other expensive hobbies like blasting clay pigeons and real pheasants with bespoke Purdeys, Berettas, etc.. it's not that difficult to get a sporting shotgun licence.
Very easy. You live in a third world country where home made guns abound. Hell, they can do it in Africa. They bang out amazingly good ones in Peshawar, but have yet to invent reticulated potable water supply or the flush toilet there.
Pretty sure the UK has people capable of doing basic machining. These are 'solved problems'.
probably more like to convert replica guns or decommissions guns to crudely made but deadly working firmarms, than making guns from scratch
Guns converted using basic tools bought on eBay - Telegraph
crudely made firmarms can be easily made, even convicts have improved basic materials to a working gun which is only effective at very close distance
all professional made guns have twisting groove inside the barrel to ensure the projectile path to be straight and accurate, this is very hard to do with a basic lathe and machines