Depending on how large the school is and how distinctive the uniform is, it may not be commercially viable for mainstream suppliers to produce/sell it.
Dd1's state secondary has a v. distinctive uniform in a colour that is not readily available in other shops. You have to buy it from one specified uniform shop somewhere in the arse end of nowhere, and of course they can charge the earth cos there's no competition. Blazer is £80 or so, kilt, shirt, jumper, compulsory winter coat £90+, school logoed PE kit, separate summer uniform, yadda yadda. If you bought the whole lot new you wouldn't see any change from £400.
BUT I know the school have tried to interest other uniform suppliers (John Lewis etc) and been told that it's not economically viable for them to take on because the items are distinctive to that particular school, which has an intake of only 90 kids a year, ie they couldn't make a reliable profit on it. A different nearby school with a similar fancy uniform (kilts, blazers, fancy PE kit) but in a more mainstream colour and with an intake of 180 girls a year is significantly cheaper in terms of uniform, even from similar shops.
So unless the school radically changed their uniform to something much more mainstream (which would be very unlikely) there's v. little parents can do about it (apart from buy second-hand - lots of parents do that, which presumably squeezes the potential profit margins still further).
Tbh, I think the prices you've been quoted are not unreasonable. Once you've had a look at the items you can work out for yourself which things are indistinguishable from the M&S version, but I doubt it would be much cheaper in any case.