I rang the Disney UK number today and booked all of my ADRs with no probs!
I called 0870 5454900 and it's option 3 for dining but that option only becomes available after 12 noon (UK time)
I was on hold for about 3 - 4 minutes then had everything booked within 20 minutes - and a lot of that was the policies and safety info as I also booked the trail rides, oh and a bit of complication as my name is changing before the trip

but credit card will still be in maiden name & ID is needed for dining in the Castle - all sorted but obviously made my call a little longer than some!
Minor time changes and 1 day change but other than that a very smooth process!
Good Luck!