I'm a florist in central London at a reasonably high end place. It does seem a bonkers amount of money ( that said, I did a wedding last year where the flower bill was £35,000) but I think you are being slightly disingenuous by comparing them with M&S flowers which ( professional opinion) can be dreadful. It's coming up to the time of year for me when I spend a lot of time rescuing the displays that people have ordered from supermarkets and been disappointed with on arrival.
Your florist should have discussed with you what varieties she was planning on using- out of season will cost more, certain imported varieties will cost more, having your wedding around Valentine's Day, Mother's Day or Christmas will massively inflate the prices because the cost price goes up. You could drill down into this a bit more and ask for a breakdown of what's being spent where - a good florist will be able to account for their pricing structure without hesitating.
Personally, we don't charge any extra for wedding flowers, but the way we work out your quote relies on quite a few factors. I suspect this is the same for most florists, which is why people perceive a ' wedding tax'.
First of all, if you are using a 'recommended supplier' that a venue insists you use, your flowers will be approximately £20% higher. That 20% goes straight to the venue as their fee for putting work your way. If your contracts states you have to use that supplier, tough luck. You'll get poor value for money, because the florist is also expecting to make their regular margins after the 20% has been accounted for.
After Brexit we anticipate that the price of flowers will go up by at least 25%, so that may well have accounted for a hike in price. Lots of florists I'm talking to haven't thought this through yet, but in talking with our Dutch wholesalers they fully anticipate this being the case. British flowers are good, but the market isn't as advanced as Holland's, and won't be able to take the strain of supplying the UK's entire flower needs. It's also very season reliant, so no having peonies in November, for example.
Most supermarkets own their own farms in Kenya/ Ecuador and so control the means of supply; thus they are able to keep their prices low. It's a common conception that Florist prices are high- they aren't: Supermarket prices are artificially low.
There is also the cost of transport and set up to take into consideration- a wedding at two venues ( church and reception) plus delivery of bouquets to one address and buttonholes to another takes time- I have to hire a second driver on days of big or multiple weddings - that price has to be factored in somehow - the price of a second florist if the venue needs dressing in a way that I wouldn't expect the driver to be able to is sometimes necessary. Hire of items: we don't have an infinite amount of every container in every size and finish in stock so might use a hire company for particular things like large urns.
I've also factored in not just my time to make your arrangements ( what you have described OP would take an entire day, and my time isn't free: is yours?) but also the several meetings I would have had with you to discuss what you want, the emails, the mood boards, the time spent on the phone to Holland finding out what's available, adjustments to the quote etc. All this stuff has to be factored in- the price of the individual stems doesn't cover this. Supermarket flowers arrangements are largely made piecemeal by people on minimum wage working in massive sheds from a photocopied picture. They don't have the training or expertise to be able to do my job, which is part of what you pay for if you go to a florist.
Finally, I order in extra flowers for your wedding so I can select the very best ones for your displays: not all roses in one wrap are open to the same point, for example; some come in bruised. I allow for this when doing my pricing.
As an extra, but possibly most important point, not all flower arrangements are created equal - I've seen some absolute shockers over the years from other weddings that I wouldn't have allowed out of the door of my shop and would have sacked the person who made them, but people profess to be thrilled with. In fairness, there are some fucking awful florists out there too, just like in any walk of life.
I am extremely good at my job - I've been doing it for 30 years and I try and give people good value for money, although I accept that means different things to different people. Flowers are a luxury, not a necessity. It's slightly wearing that people think florists are 'a rip off' in a way that no one seems to think, say, restaurants are. I know how much a potato costs, but I don't berate somewhere for charging me £3.50 for a side of chips.
Ultimately, without seeing the quote and its breakdown I can't say how unreasonable it might be. I'm just giving you an insight into why, on first glance, flowers seem expensive.