I have looked for and can't find a flow straightener. This might not be the correct name, it may be known as a spa or an aerator. It screws onto the end of the tap spout, in this case a bath mixer tap. It is round and made of plastic.I have been told that I could get one from the manufacturer of the tap, but I don't know the make of the tap. All the DIY sheds, plumbing merchants and hardware shops I have tried don't sell them.
The item has a diameter of 1 3/16" (3cm) and the thread diameter is 1 3/64 (2.65cm). The thread is either 1.25 AF, 20 Metric, 19G Whitworth or something very close to one of these.
I am in the UK. This is my first posting to this group. Let's see how good it is.