Hi Neal,
Please check and increase Quotas as needed so that you have enough to complete the migration. There are two you need to check, one for the specific instance family size for uksourth and one for Total Regional vCPUs for uksouth.
Below is the link to where you can view and request increase of Quotas:
https://portal.azure.com/#view/Microsoft_Azure_Capacity/QuotaMenuBlade/~/myQuotas
Please click Accept Answer and upvote if the above was helpful. If you are unsure add a comment and I will assist further.
Thanks.
-TP