People shouldn't assume that using weapons is currently something that everybody can do. Even if somebody is hell bent on causing injury to somebody else, and has time to plan their actions, it's still often easier to use a knife or other weapon than a gun. And, again, other weapons are easier to survive, easier to escape from (you can run away from a knife, but running away from a gun isn't much use) and, in many cases, easier to defend against.
Guns are only great levelers if both sides have one. I can't see everybody in the UK buying guns if they were legalised, and the people most likely to want them first are the people who expect to need them (ie, criminals).
Regarding pepper spray, I think the people who are currently most likely to use it are unfortunately criminals. Also, people who don't know how to use pepper spray can quite easily incapacitate themselves with it (gust of wind at the wrong moment, spray ''splashback'' etc) making it a risky last resort even if somebody did use it for self defence. Also, I don't think that enough people know what self defence actually is.
I don't think that a farmer shooting a burglar is necessarily self defence. The burglar would have had to have shown a genuine threat to the safety of the farmer or others that justified killing them. To stick up for a farmer shooting somebody (who, in the cases I've heard about, gave their killers no reason to believe that they were armed or about to become violent) and then criticise armed police for shooting a very small minority of the known dangerous criminals who they come into contact with seems unfair, to me at least.
Armed police in the UK have a very good record, overall. C019 in London shoot people twice a year, on average, and I only know of two police shootings since 2008 where the justification was in any doubt (Rodney and Duggan). Armed police have to requalify every six months in most forces, and (because there are so few armed criminals or civilians) aren't usually required to have more than a sidearm with them on duty unless there's a specific threat. I know the police get a lot of criticism (and rightly so) over those two shootings, but overall I would be happier to know that there was a police officer with a gun nearby than an unknown number of civilians or criminals with guns nearby.
I know some people would like to have hunting rifles, and I know several people who do. It is possible (one of them even lives in London, although he has to store the rifle out of town) and I don't think that there is a problem with current firearms laws in the UK in that respect.
The current definition of self defence in UK law is a bit vague, but I don't think that it's a huge problem. Generally it seems to be applied sensibly, and all laws are open to different levels of abuse. In a way, having such vague wording makes it harder to take advantage of technicalities.