There's a lot of good advice in this thread, it's a long one and I'll admit I've only skim read it so apologies if I repeat what others have said.
I did see a few people mention the canonical and also WordPress and potentially moving the site across.
From a technical SEO perspective, you DO need to remove the canonical tag which is pointing back to your homepage. I suspect this is is a fixed line in your header template and can be removed from all affected pages with a single deletion.
You should move to WordPress, as already mentioned by someone else, your assumptions about it are wrong. You can get hosting with SSL with a Namecheap for about £30 for the whole year, whenever I take on a new client I normally do this for them and it can save a fortune. The page render time is an SEO task to improve if it's poor by default.
https://docs.google.com/spreadsheets/d/1vxN4jbHuoqHu23Q8_HoLje_hFVQEbeS4ZvT1IQRY134/edit?usp=sharing I've downloaded your current keyword rankings as per Ahrefs just to give you an idea of where your current low hanging fruit opportunities are.
As others have mentioned, if you don't already, then you should get yourself onto Google Analytics and Search Console. GSC will allow you to see the exact search queries people are using to find your pages along with the clicks and impressions, you can use this data to then re-optimise your pages using the correct data rather than third party tools such as Ahrefs.
Moving to WordPress will make it a lot easier to sort out your technical issues, things like your website is using www but the non-www version does not redirect
https://i.imgur.com/xM468c7.png your canonical is using www so this is taken care of but it does mean that some of your backlinks are pointing at the non-www version (e.g.
https://cpdonline.co.uk/career-guides/how-to-become-a-bailiff/) so you're losing link equity that way.
You have other issues too such as some pages have multiple H1 tags and whereas this isn't necessarily a bad thing, SEO best practise would be to have a single one and have your main keyword in it.
I'm also seeing some broken links in places, again not good for technical SEO.
Your website is aged, seems like April 2018 it was first registered and according to Ahrefs you seem to have weathered many Google update storms quite well with no sudden dips in your traffic graph or abnormal backlink acquisition peaks or troughs.
In short, I think the age has helped your site and Google has a certain amount of trust with you but if you fix the technical issues and then use data to optimise your pages based around what you can prove people are actually searching for you should see further improvement.
Take into consideration what others are saying here too with regard to your layout and design. Know which device your audience are using (likely mobile/tablet in a B2C environment) and design your UX around that whilst making sure not only do you meet the user intent but also provide them with your ultimate Call To Action in a way that works for both you and the user.