That's an interesting and non-simple question. The block is self-imposed, Ofcom didn't do that, Josh did. He also voluntarily posted the letter he received. It's entirely possible that Reddit has received a similar letter. The letter in itself doesn't actually carry any enforcement and the UK govt. hasn't yet figured out how or when they'll start enforcing it (that they've disclosed).
Lets face it, Josh has a rather contrarian disposition (site would be long gone if he didn't). He's taken great exception to our government presuming to tell him and his country what they can and can't host. This letter is at most a prelude to action. They may even be regretting the attention he has drawn to it.
For interest, here are Ofcom's pages about it:
Quick guide to illegal content risk assessments - Ofcom
Risk Assessment Guidance and Risk Profiles
The second one which contains the full guidance is the one with the fine detail. There's guidance in there which would raise hackles on the Farms even if there was a willingness to comply with another country's laws. Anonymous users are listed as a risk factor. Well anybody openly posting on the Farms under their own name is a rarity. Caroline Farrow being the most notable open poster, though there are others.
Though having skim-read through it a lot of the guidance actually wouldn't be a problem in practice. Like there's a firearms section about trading weapons but nobody buys or sells anything on the Farms. The thing is though, he'd still have to go through every "risk factor" like this filling out the section. I really don't see Josh of all people going through an 84 page form at the required annual frequency (or ever). It doesn't help that they're requiring him to submit his assessment by the 16th of March this year and the letter is dated the 26th.
The site and guidance generally reads well-intentioned to me. I can give the benefit of the doubt to the people who wrote it that they were genuinely trying to solve the problem of dangerous content online. And that they had many a civil service meeting to think up all the things that should be checked. But the outcome is something broadly unworkable and we've seen small UK forums voluntarily shut down because they feel they either can't comply or that they feel this is putting the legal burden on them for what users do on their services.
It's not bad for a forum to have measures in place to prevent harm. And if someone posts something illegal on the Farms or calls for illegal actions, the userbase itself is pretty pro-active about shutting that down quickly. (They call inciting illegal behaviour "fedposting" under the belief that it's likely a Fed trying to provoke someone into agreeing). But there's a line between having measures in place to make sure your site is safe and being legally responsible if someone posts something they shouldn't. Big players can handle that. Smaller sites can't.