I suspect some items don’t appear in stock (both DE and now UK), but the DS agents seem able to source them.
I just made a parts order from the UK site, my first in quite a while. In the past I have sent them a spreadsheet and they have sent me back a link to pay, sometimes with a list of items not available. That worked fine, despite the odd mistake in part numbers (more often at their end than at mine tbh).
Yesterday evening (Sunday) I emailed a list of 28 items which I estimated would cost over £40 and therefore be free postage. At 8:55 this morning I had a message saying parts could now be ordered via the website (i.e this is now the only way.). I ordered some of the parts via the website and paid with PayPal, including £3.50 postage costs. I sent a message to customer services asking what could be done about the items I could not order on the website. I received a message back "We can sometimes order parts that are not available for you to order on line. I have booked these on for you and will email a link to pay."
Of course, there was another £3.50 postage to pay. But the response was very prompt, presumably because PM staff are no longer spending all their time entering part numbers!
In the past, I have found the UK DS spare parts service really excellent, but I am not keen on this new system:
Pros:
fewer errors in orders placed,
quicker response for items ordered online (but still probably a long wait before items are delivered),
less tedious work for PM staff,
you can combine orders for parts and sets (I think you can do this)
Cons:
postage costs may be higher (double in this case), may be harder to avoid postage costs,
online Shopping Cart may auto-empty if you delay completing the order,
tedious entry of part numbers unless you have an app. or code to do this for you,
fiddly to enter quantities larger than 1 of an item,
still need to rely on PM staff to order some items, though that's not a big problem really
still need to keep lists of required parts before making an order