If Izzard doesn't have ADHD I'd be pretty amazed. Izzard is able to hyper focus on tasks and issues he likes and take them to the extreme. So he can work hard, but lacks the discipline / ability to work on the details, prefering to wing it and improvise when it suits his attention span. Its not that he's unintelligent - he clearly is intelligent but he can't do coherent because his thought jump around all over the place - which was great for his style of comedy but not for forming powerful political arguments.
More than this for the everyday slog of dealing with constituents who have various and often rather dull issues, he'd be lost and just wouldn't bother because it just doesn't grab his interest and attention.
He reminds me a great deal of Sheffield Hallam's previous MP Jared O'Mara in many respects. He was picked in haste by the Labour Party in an emergency selection process due to a snap election - his identity as disabled being one thing that favoured his choice - and then won Nick Clegg's seat in a result that shocked everyone. His time in Parliament can best be described as a 'total car crash' to put it mildly as it quickly became apparent he was totally unsuited to the job.
I think if Izzard did become an MP, he'd become a total liability and distraction for the Labour Party at some point, because he'd be unable to keep his gob shut when appropriate / expected as he has a reputation for running his mouth off. He's become the story rather than issues being the narrative. No leader is really going to want that if they've got any sense.
Izzard has been going through the normal selection process at present. As it stands, the Labour Party probably will have selections all done by the time of a GE - expectations are to be ready by April / May 2024 - rather than being caught by surprise and need to go through an Emergency Selection Process like in 2017, but I'd still keep an eye on if an election is called suddenly. Thats when there may be a chance to stand in a constituency which hasn't got a candidate might crop up at the last minute.