Is it a one off job?
A silverline special would be fine if so, a couple of quid from Toolstation. You are cutting both the rebate and the tongue so you can use imperial or metric whatever they have. The size is flexible to a degree too if they only have 8mm, etc.
As said, a saw blade could be used although for a flat bottomed cut the router would give a better finish (depending on the tooth configuration of the blade)