I have read this thread with interest, I ran a web hosting company back in the days when PHP was new. We were one of the first serious hosts to offer PHP as a proper service with a control panel for resellers etc.. since then I have managed some large ecommerce companies using everything from osCommerce, to Magento to now Shopify and WooCommerce.
I currently manage Shopify and WooCommerce sites for clients who clear well over £1,000,000 a year as well as a few smaller sites that only sell a couple of thousand a month. My advantage is the background in not only coding but also the ecommerce side of things, I have a wide understanding of issues. The latest Shopify store I migrated didn't loose any Google rankings and has eliminated 99% of management and customer frustrations (can't please everyone all of the time)
I would disagree with several comments above, sorry if this ends up a long message
Shopify
Shopify is a fantastic platform with some caveats, some features are restrictive and cannot be overridden. However they are working on several new features that are due to be rolled out this week which will allow some of these restrictions to be lifted.
It's secure, when a theme needs updating, you don't have to do it instantly, it can wait a little while, unlike Wordpress where if you don't update you can be hacked. This is the benefit of a SaaS
.
The downsides are restrictive checkout (although this is one of the changes coming), no support for things like quantity breaks (the apps are a fudge), theme updates are a manual process which depending on the customisations can be a headache (again new features should remove some of the pain of customisations). The other frustration I find is the lack of mixing of discount types.
It's all workable but is a pain, however it just works, the interface is admin friendly, customers love it, it increases conversions if built currently and it's affordable.
The cost of apps is a potential issue except that 90% of the time most are not needed if using a decent theme with a decent developer. I recently migrated a highly customised Woocommerce store to Shopify, we didn't use one paid plugin. I utilised the theme and custom coded everything else. It didn't cost the client anymore as I had costed it. Their overall costs of management have halved now.
Not all clients are suited, usually due to initial budgets and potential size.
I see so many people who have setup a Shopify shop or paid someone on fivver and it's terrible, it won't convert and either looks spammy or just thrown together.
All I would say is if you are ready to switch is be aware of the new updates to the themes Online Store 2, it's going to be a major overhaul for Themes and apps and Themes have to be upgraded by the end of the year.
Woo/Wordpress
A brilliant platform and very flexible but one that isn't maintained is a website waiting to be hacked. Quite often updates get left and by the time everything needs updating you end up with a crashed website.
We have had two in the last two months that needed a full rebuild due to this.
It has flexibility but often due to the backend being intimidating clients are scared to touch anything.
Automated updates are fine until you have some custom work or something goes wrong, doing these by hand doesn't take long and allows someone to physically run through the site to make sure features function correctly.
This platform suits a lot of clients but if not managed correctly will do more hard then good.
OP
The bit that jumps out is if £100 a month could use up your profit then you 100% don't need a VPS. ecommerce is not cheap, in the same way running a physical shop isn't cheap.
- All future minor changes to the website design and functionality
What does this even mean, minor changes could be anything from a phone number to a text change. Minor is fairly loose
- All bugfixes and technical support for the website.
Most sites don't need bug fixes, they need updates.
- All security updates for the website.
So that's an update of Wordpress which is usually a click of a button
- Private server hosting – a virtual private server in our datacentre dedicated to your website.
VPS servers are often billed as the be all and end all, most of the time with cheap hosts they are not setup very well and are not true VPS's
- Server side maintenance and security updates.
This would be provided by the hosting company anyway, they don't want hacked servers
- Email hosting for your chosen emails.
Office365 is one of the best email services if you don't want your emails being flagged as spam, if they are any good they will be providing this, if not it's usually locally hosted emails and should be avoided
- Access to our development team, design team and legal team for all matters around the website.
Several days to reply isn't what you need/want with ecom
- Dedicated account manager working on your behalf with all the teams above (myself).
- Access to our partners for SEO, SMO, PPC, content marketing and off-site marketing at discounted rates should you require them.
Worthless
It all sounds great but I suspect you are paying over the odds, a decent hosting package with less fluff and a direct support contract that is specific is better suited. The above doesn't really offer you anything, £100 for that is a lot of money in my opinion.
Ok sorry for the long message and dumping my opinion on you.
I think we all agree that you are probably paying too much 
Good luck
Darren