Is Glasgow filled with some sort of protected class? Lot of Africans, or Muslims? Was the joke meant to be racist?
No more than any other major city. Anecdotally I'd say far less so than the south of the UK, but I don't have numbers to hand, so take that as opinion rather than fact.
Or just anti-life?
Basically, this. It could be construed as "classist", as Glasgow's primary reputation is being something of a rough city (or a tough/hard city from a local's point of view). - however the irony isn't lost on me that Sunderland (where the 'tweeter' resides) isn't exactly god's kingdom either. An argument could also be made that Scots themselves are a minority, having only ~10% of England's population. (Easy to miss, the accident occurred in Scotland but the accused is in the north of England).
But mostly, he just seems to be a dick. And that's much less protected here than it is in the US.
This is one of the biggest differences between the US & UK legal cultures. While basically similar systems (they share the same "common law" roots), they're exercised entirely differently. The US deals very much with absolutes, and the letter of the law. Most of your primary rights are the right to dissent - and understandably so, since it's what your nation was built upon. To this end you have free speech, press, assembly, firearms, etc. That is, the right to have a dissenting opinion, to share/publish this opinion, to vote on it, to protest, etc. All the way up to having enough guns around that the govt should be wary of the people - although personally I'd argue this one's now a futile effort given the govt clearly won that arms race
But I digress; In the UK the law is very much more intent based. So we don't have free speech as an absolute - we judge each on its merits. So political speech, satire, etc are essentially protected speech - very much in line with the intent of your first amendment - but public disorder, breech of the peace, hate speech, etc, very much less so - which runs contrary to the letter of your first amendment.
This one falls into an odd gray area though. We have no obligation to provide a platform for free speech. So while this wouldn't have been seen has malicious if shared as a joke between friends, it falls foul of the Malicious Communications Act, which allows for something like;
Section 1 of the Act covers the sending to another of any letters, electronic communications, photographs and recordings that are indecent, grossly offensive or which convey a threat (which may be false), provided there is an intention on the part of the sender to cause distress or anxiety to the person who receives them.
The offence refers to the sending, delivering or transmitting, there is no requirement for the communication to reach (or be read by) the person who is intended to read it.
So this is where we end up with situations which can appear absurd. Someone makes a complaint to the police. The police satisfy themselves that there's reasonable grounds that such a communication has been sent. But it's not up to the police to judge the intent ("provided there is an intention on the part of the sender
So far, only the first of these steps has happened.