Marks and Spencers started as a department store; they still have this operation. They sell clothes, beauty products, cookware, homeware and furniture. All these things are sold in physical shops and online. Most of this is straightforward for an e-commerce operation, but the furniture will involve separate warehousing and delivery systems.
They also offer financial services (bank accounts, credit cards and insurance). These are white labelled products, but they are closely linked to their loyalty programme (the Sparks card).
Finally, they have their food operation: M&S is also a high-end supermarket. You can't do your food shop on the M&S website (although their food products are available from online-only supermarket Ocado), but you can order some food products (sandwich platters and party food) and fresh flowers from the website.
So M&S is a mid-tier department store and a high-end supermarket. These are very different styles of retail operation: supermarkets require a lot of data processing to ensure the right things get to the right shops at the right time to ensure that food doesn't go to waste but also shoppers aren't annoyed by the unavailability of staples like bread and milk.
Finally, M&S is traditionally fairly strong in customer service; it's not exactly Harrod's or Fortnum and Mason's, but their bra-fitting service, for example, has a legendary reputation. The internet isn't their natural home.
So all-in-all, you have a business doing complicated things online because they think they have to, not because they want to: a pretty clear recipe for disaster.