I act from conviction
You are stubborn
S/he is a fanatic
Corbyn is not proposing to replace capitalism … he is not even proposing very significant wealth distribution….according to the economist James Medway, the economic proposals in Labour’s 2017 manifesto were more conservative than those put forward in 1983 by the SDP – the breakaway party formed by a faction of Labour’s right wing. Corbyn Now, Lorna Finlayson, London Review of Books, 27/9/198
What is the difference between and extremist and a moderate point of view? Where is the famous ‘middle ground’?
It depends, of course on where you are standing at the time.
A woman in the early 1900s, asking for the vote, or a slave asking for freedom, might be considered to be making a moderate claim by our standards, but someone wishing to keep then subdued will always call it an extreme claim. What is called ‘extreme’ tends to change over time, and the word is always a useful insult to try to demonise your opponent so you can ignore them.
Conversely, everyone wants to vote for ‘moderate’ policies, so if you can get your extreme policies relabelled and perceived as moderate that always helps.
In the context of the British political system, it is widely accepted that the centre ground moved to the right from the late seventies onwards. Ideas and habits of thought that were previously considered radically right wing became accepted as normal. What was previously seen as a moderate or consensus point of view became demonised as left wing or extreme.
The new ‘political map’ remains in force today. Anyone trying to move the centre back to where it used to be is thus labelled an ‘extremist’ by the right wing movements trying to lay claim to the label centre of moderate, to persuade voters to follow them.
For example, Richard Ashworth was leader of the Conservative group in the European Parliament under David Cameron but in October 2018 was expelled for defying his party line on Brexit. He now claims his party has drifted into “far-right ideology and intolerance”. The Hungarian government, widely considered by European MPs to be intolerant and extremely right wing, could have been censured for their record of “antisemitism, Islamophobia, crackdowns on freedom of expression, and abuse of the rule of law” but the UK government, needing their support, blocked the move and threw out the MPEs who supported it. 
A month before, there were reports of right wing activists trying to join the party to get votes to elect Boris Johnson as leader and keep the party on the right. In Dec 2017 Michael Heseltine, former Deputy PM and Tory peer, claimed a Corbyn government would be preferable to the Brexit his party was pursing, implying that extremists within the Tory party were more damaging than his opposing Labour party.
In October Sarah Wollaston, a long standing Tory MP in Totness, found herself facing deselection because of a rush of new members joining to oppose her ‘moderate’ views on Brexit. Interestingly, in the same newspaper, a few pages apart, Theresa May was claiming her was the party of moderate views and Labour supporter should join her to avoid what she called the extremists who has taken it over. That word is a weapon everyone uses and to work out how it is misused we need to look at some examples of historical change.
If Labour is so extreme, who do Tories copy and support them?
As long ago as 2007, George Osbourne (then Chancellor) was courting popularity by saying he would match Labour’s projected spending plans – not condemn them but copy them.
In September 2018 Jeffrey Archer, former deputy chair of the Tory Party, said if he lived in the north of England he would vote for Corbyn.
Labour constantly argued that we should build more council houses. After the Tory Party conference in 2018 Theresa May lifted the spending cap so councils could borrow more money to do so.
The fact is, the Tory party has pushed UK politics to an extreme position and many Tories are busy trying to push it back to the centre, where Labour has always been.
 Observer 7/10/18