It's better not to make eye contact, or reply to any racist taunts or remarks. Right?
Note well the Bystander Apathy Effect. Even if I yell for help, people may not assist especially if they're racists too!
I keep walking, but these racists follow me. I'm 51 and can't out-run these chavs for long.
I'm too far from a police or fire station.
Then what do I do? Even if I find a fire extinguisher, spraying it at them won't cripple them.
First of all, let's emphasise that this remains the actions of a very small minority of people. The media have a general tendency to amplify things. If you look at reporting of events in many places, you sometimes have the impression there's a widespread civil war going on when there are actually only small isolated incidents.
Then, you need to distinguish the type of abuse. The first situation is verbal abuse. It is of course not something you want to deal with, but at least there's no physical harm. I'd say you should not confront the authors of that abuse (to avoid getting into the physical harm part below). Walk away and report it to the police.
If it comes to threats of physical harm, again, try to de-escalate the situation, then report it to the police. You could tell your aggressors that if they touch you they incur the risk of catching said virus (even though it's most probably completely false, but those people are stupid enough), so they should refrain from doing so.
The emergency number is 999 or 112 from a mobile. If you need help, call them. If you need help from bystanders, be direct. Don't say "someone please help", designate someone and instruct them to call 999.