"The UK had no beef in this originally, they were just acting on an EU arrest warrant"
They acted on an invalid EU warrant. They post-fixed the paperwork later when he was in solitary. And why was he unnecessarily put in solitary? There were so many things wrong about the way the UK went about it.
Bail-jumping was a serious thing to do but Assange had weighed up his options. The risk of being executed evidently outweighed being cooped up in a hole for years. He was pretty much screwed either way.