I've an 16amp 250v appliance with an EU plug. Would this adapter be safe to use for it?

Standard UK/Ireland socket and travel adaptors are usually rated to 13 A, 240 V (about 3 kW max).

That means any adaptor that is only rated 13 A is technically underspecified for a device that can draw 16 A at full load. It may work in practice if the appliance never reaches that current, but from a safety and compliance standpoint, you should match or exceed the appliance current rating.