What can we do about App Service capacity in UK South?

Try escalating through your Microsoft account manager as well as the support ticket. Capacity has reportedly been granted in some cases despite other requests being backlogged, so ask them to check the specific SKU, subscription and deployment requirements.

Don't assume an escalation guarantees anything. Ask for a clear status and what alternatives they can actually allocate now.

UK West may be an option if your workload can run there. One workaround is to put the App Service Plan in UK West and use a private endpoint in UK South to connect it to a UK South virtual network.

That still means the app runs in UK West. Check your data residency requirements and test the cross-region latency before relying on it.

Check existing subscriptions before creating a new one. Some existing subscriptions may already have usable quota while a new subscription does not. Confirm the available quota for the SKU and region, then test whether the deployment can actually be made.

Look beyond the immediate deployment too. Other European regions have also reportedly had capacity constraints, so a fallback region isn't necessarily available just because it worked before.

For disaster recovery, check whether the recovery plan depends on creating fresh capacity after an outage. It's worth discussing that risk and any reservation options with Microsoft rather than assuming capacity will be there when needed.

If the workload can be reworked as an Azure Function, ask whether Flex Consumption is a viable alternative. It isn't a drop-in replacement for every App Service workload, and you'd still need to check regional availability and fit.